diff --git a/main.py b/main.py
new file mode 100644
index 0000000..c48b597
--- /dev/null
+++ b/main.py
@@ -0,0 +1,296 @@
+import sys
+from pathlib import Path
+from typing import Optional
+
+from PyQt6.QtCore import Qt, QTimer, QThread, pyqtSignal
+from PyQt6.QtGui import QShortcut, QKeySequence
+from PyQt6.QtWidgets import (
+    QApplication, QMainWindow, QWidget, QVBoxLayout, QHBoxLayout,
+    QPushButton, QLabel, QSlider, QCheckBox, QDoubleSpinBox, QFileDialog,
+    QMessageBox, QPlainTextEdit, QGroupBox,
+)
+
+from player_widget import PlayerWidget
+from sync_logic import compute_offset, SyncOffset
+from ffmpeg_export import export_synced, FfmpegExportError, ExportResult
+
+
+def format_timestamp(seconds: float) -> str:
+    m, s = divmod(max(seconds, 0.0), 60)
+    return f"{int(m):02d}:{s:06.3f}"
+
+
+class ExportWorker(QThread):
+    finished_ok = pyqtSignal(object)   # ExportResult
+    finished_err = pyqtSignal(str, str)  # message, stderr
+
+    def __init__(self, input_path, delay_ms, output_path):
+        super().__init__()
+        self.input_path = input_path
+        self.delay_ms = delay_ms
+        self.output_path = output_path
+
+    def run(self):
+        try:
+            result: ExportResult = export_synced(self.input_path, self.delay_ms, self.output_path)
+            self.finished_ok.emit(result)
+        except FfmpegExportError as e:
+            self.finished_err.emit(str(e), e.stderr)
+
+
+class MainWindow(QMainWindow):
+    def __init__(self):
+        super().__init__()
+        self.setWindowTitle("A/V Sync Marker")
+        self.resize(960, 720)
+
+        self.input_path: Optional[str] = None
+        self.video_mark: Optional[float] = None
+        self.audio_mark: Optional[float] = None
+        self.offset: Optional[SyncOffset] = None
+        self._export_worker: Optional[ExportWorker] = None
+        self._slider_being_dragged = False
+
+        self.player = PlayerWidget()
+        self.player.setMinimumHeight(400)
+
+        self._build_ui()
+        self._wire_shortcuts()
+
+        self._position_timer = QTimer(self)
+        self._position_timer.setInterval(100)
+        self._position_timer.timeout.connect(self._update_position_ui)
+        self._position_timer.start()
+
+    # -- UI construction ---------------------------------------------------
+
+    def _build_ui(self):
+        central = QWidget()
+        layout = QVBoxLayout(central)
+
+        layout.addWidget(self.player, stretch=1)
+
+        # Transport row: play/pause, frame step, seek slider, position label
+        transport = QHBoxLayout()
+        self.load_button = QPushButton("Load File…")
+        self.load_button.clicked.connect(self._on_load_clicked)
+        transport.addWidget(self.load_button)
+
+        self.play_button = QPushButton("Play")
+        self.play_button.clicked.connect(self._toggle_play)
+        transport.addWidget(self.play_button)
+
+        self.prev_frame_button = QPushButton("⏮ Frame (,)")
+        self.prev_frame_button.clicked.connect(lambda: self._step_frame(-1))
+        transport.addWidget(self.prev_frame_button)
+
+        self.next_frame_button = QPushButton("Frame ⏭ (.)")
+        self.next_frame_button.clicked.connect(lambda: self._step_frame(1))
+        transport.addWidget(self.next_frame_button)
+
+        self.mute_checkbox = QCheckBox("Mute audio")
+        self.mute_checkbox.toggled.connect(self._on_mute_toggled)
+        transport.addWidget(self.mute_checkbox)
+
+        layout.addLayout(transport)
+
+        seek_row = QHBoxLayout()
+        self.position_label = QLabel("00:00.000")
+        seek_row.addWidget(self.position_label)
+
+        self.seek_slider = QSlider(Qt.Orientation.Horizontal)
+        self.seek_slider.setRange(0, 0)
+        self.seek_slider.sliderPressed.connect(lambda: setattr(self, "_slider_being_dragged", True))
+        self.seek_slider.sliderReleased.connect(self._on_slider_released)
+        seek_row.addWidget(self.seek_slider, stretch=1)
+
+        self.duration_label = QLabel("00:00.000")
+        seek_row.addWidget(self.duration_label)
+        layout.addLayout(seek_row)
+
+        # Marker row
+        marker_group = QGroupBox("Sync markers")
+        marker_layout = QHBoxLayout(marker_group)
+
+        self.mark_video_button = QPushButton("Mark Video Event")
+        self.mark_video_button.clicked.connect(self._mark_video)
+        marker_layout.addWidget(self.mark_video_button)
+        self.video_mark_label = QLabel("video: —")
+        marker_layout.addWidget(self.video_mark_label)
+
+        self.mark_audio_button = QPushButton("Mark Audio Event")
+        self.mark_audio_button.clicked.connect(self._mark_audio)
+        marker_layout.addWidget(self.mark_audio_button)
+        self.audio_mark_label = QLabel("audio: —")
+        marker_layout.addWidget(self.audio_mark_label)
+
+        layout.addWidget(marker_group)
+
+        # Offset + export row
+        export_group = QGroupBox("Offset and export")
+        export_layout = QHBoxLayout(export_group)
+
+        self.offset_label = QLabel("Offset: — (mark both events)")
+        export_layout.addWidget(self.offset_label, stretch=1)
+
+        export_layout.addWidget(QLabel("Override (ms):"))
+        self.override_spinbox = QDoubleSpinBox()
+        self.override_spinbox.setRange(-600000, 600000)
+        self.override_spinbox.setDecimals(0)
+        self.override_spinbox.setEnabled(False)
+        export_layout.addWidget(self.override_spinbox)
+
+        self.export_button = QPushButton("Export…")
+        self.export_button.setEnabled(False)
+        self.export_button.clicked.connect(self._on_export_clicked)
+        export_layout.addWidget(self.export_button)
+
+        layout.addWidget(export_group)
+
+        self.log_view = QPlainTextEdit()
+        self.log_view.setReadOnly(True)
+        self.log_view.setMaximumHeight(120)
+        self.log_view.setPlaceholderText("ffmpeg output / warnings will appear here after export.")
+        layout.addWidget(self.log_view)
+
+        self.setCentralWidget(central)
+
+    def _wire_shortcuts(self):
+        QShortcut(QKeySequence(","), self, activated=lambda: self._step_frame(-1))
+        QShortcut(QKeySequence("."), self, activated=lambda: self._step_frame(1))
+        QShortcut(QKeySequence(Qt.Key.Key_Space), self, activated=self._toggle_play)
+
+    # -- transport -----------------------------------------------------
+
+    def _on_load_clicked(self):
+        path, _ = QFileDialog.getOpenFileName(self, "Load video file", "", "Video files (*.mp4 *.mov *.mkv *.avi *.webm);;All files (*)")
+        if path:
+            self._load(path)
+
+    def _load(self, path: str):
+        self.input_path = path
+        self.player.load(path)
+        self.video_mark = None
+        self.audio_mark = None
+        self.offset = None
+        self.video_mark_label.setText("video: —")
+        self.audio_mark_label.setText("audio: —")
+        self.offset_label.setText("Offset: — (mark both events)")
+        self.export_button.setEnabled(False)
+        self.override_spinbox.setEnabled(False)
+        self.log_view.clear()
+
+        duration = self.player.mpv.duration or 0.0
+        self.seek_slider.setRange(0, int(duration * 1000))
+        self.duration_label.setText(format_timestamp(duration))
+        self.setWindowTitle(f"A/V Sync Marker — {Path(path).name}")
+
+    def _toggle_play(self):
+        if self.player.mpv.pause:
+            self.player.play()
+            self.play_button.setText("Pause")
+        else:
+            self.player.pause()
+            self.play_button.setText("Play")
+
+    def _step_frame(self, n: int):
+        self.player.pause()
+        self.play_button.setText("Play")
+        self.player.step_frame(n)
+        self._update_position_ui()
+
+    def _on_mute_toggled(self, checked: bool):
+        self.player.mpv.mute = checked
+
+    def _on_slider_released(self):
+        target_sec = self.seek_slider.value() / 1000.0
+        self.player.seek(target_sec)
+        self._slider_being_dragged = False
+        self._update_position_ui()
+
+    def _update_position_ui(self):
+        if self.input_path is None:
+            return
+        pos = self.player.get_position()
+        self.position_label.setText(format_timestamp(pos))
+        if not self._slider_being_dragged:
+            self.seek_slider.blockSignals(True)
+            self.seek_slider.setValue(int(pos * 1000))
+            self.seek_slider.blockSignals(False)
+
+    # -- markers / offset -----------------------------------------------
+
+    def _mark_video(self):
+        self.video_mark = self.player.get_position()
+        self.video_mark_label.setText(f"video: {format_timestamp(self.video_mark)}")
+        self._recompute_offset()
+
+    def _mark_audio(self):
+        self.audio_mark = self.player.get_position()
+        self.audio_mark_label.setText(f"audio: {format_timestamp(self.audio_mark)}")
+        self._recompute_offset()
+
+    def _recompute_offset(self):
+        if self.video_mark is None or self.audio_mark is None:
+            return
+        self.offset = compute_offset(self.video_mark, self.audio_mark)
+        self.offset_label.setText(self.offset.describe())
+        self.override_spinbox.setValue(self.offset.delay_ms)
+        self.override_spinbox.setEnabled(True)
+        self.export_button.setEnabled(True)
+
+    # -- export -----------------------------------------------------
+
+    def _on_export_clicked(self):
+        if self.input_path is None or self.offset is None:
+            return
+        delay_ms = self.override_spinbox.value()
+
+        input_p = Path(self.input_path)
+        suggested = str(input_p.with_name(f"{input_p.stem}_synced{input_p.suffix}"))
+        output_path, _ = QFileDialog.getSaveFileName(self, "Export synced file", suggested)
+        if not output_path:
+            return
+
+        self.export_button.setEnabled(False)
+        self.export_button.setText("Exporting…")
+        self.log_view.appendPlainText(f"Exporting with delay_ms={delay_ms:.0f} …")
+
+        self._export_worker = ExportWorker(self.input_path, delay_ms, output_path)
+        self._export_worker.finished_ok.connect(self._on_export_done)
+        self._export_worker.finished_err.connect(self._on_export_failed)
+        self._export_worker.start()
+
+    def _on_export_done(self, result: ExportResult):
+        self.export_button.setEnabled(True)
+        self.export_button.setText("Export…")
+        self.log_view.appendPlainText("Export finished.")
+        if result.stderr.strip():
+            # ffmpeg logs things like "sps_id out of range" on stream-copy
+            # of screen-recording-style sources; harmless, just surfaced
+            # for visibility rather than treated as failure.
+            self.log_view.appendPlainText("ffmpeg log:\n" + result.stderr)
+        QMessageBox.information(self, "Export complete", "Export finished successfully.")
+
+    def _on_export_failed(self, message: str, stderr: str):
+        self.export_button.setEnabled(True)
+        self.export_button.setText("Export…")
+        self.log_view.appendPlainText(f"Export FAILED: {message}\n{stderr}")
+        QMessageBox.critical(self, "Export failed", f"{message}\n\n{stderr[-2000:]}")
+
+    def closeEvent(self, event):
+        self.player.shutdown()
+        super().closeEvent(event)
+
+
+def main():
+    app = QApplication(sys.argv)
+    window = MainWindow()
+    window.show()
+    if len(sys.argv) > 1:
+        window._load(sys.argv[1])
+    sys.exit(app.exec())
+
+
+if __name__ == "__main__":
+    main()
diff --git a/scripts/smoke_test_ui.py b/scripts/smoke_test_ui.py
new file mode 100644
index 0000000..e6f7640
--- /dev/null
+++ b/scripts/smoke_test_ui.py
@@ -0,0 +1,88 @@
+"""Launches the real MainWindow, loads the out-of-sync fixture, drives the
+actual marker -> offset -> export flow through the same code the UI buttons
+call (not a copy of the logic), and grabs screenshots so it can be visually
+checked instead of just trusting the app "should" work.
+"""
+import sys
+import time
+from pathlib import Path
+
+from PyQt6.QtWidgets import QApplication
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+import main as app_main
+
+ROOT = Path(__file__).resolve().parent.parent
+CLIP = str(ROOT / "test_media" / "sync_test_OUTOFSYNC_audio+350ms.mp4")
+OUT_DIR = ROOT / "test_media" / "seek_validation"
+EXPORT_OUT = str(OUT_DIR / "_ui_smoke_export.mp4")
+
+
+def main():
+    OUT_DIR.mkdir(exist_ok=True)
+    qapp = QApplication(sys.argv)
+
+    win = app_main.MainWindow()
+    win.show()
+    qapp.processEvents()
+
+    win._load(CLIP)
+    qapp.processEvents()
+    time.sleep(0.3)
+    qapp.processEvents()
+    print(f"loaded: duration slider range = {win.seek_slider.maximum()}ms")
+
+    win.player.seek(5.000)
+    qapp.processEvents()
+    win._mark_video()
+    print(f"video mark: {win.video_mark:.3f}s -> label: {win.video_mark_label.text()}")
+
+    win.player.seek(5.350)
+    qapp.processEvents()
+    win._mark_audio()
+    print(f"audio mark: {win.audio_mark:.3f}s -> label: {win.audio_mark_label.text()}")
+
+    print(f"offset label: {win.offset_label.text()}")
+    print(f"export button enabled: {win.export_button.isEnabled()}")
+    assert win.export_button.isEnabled()
+    # hr-seek lands on the nearest frame boundary (33.3ms steps @ 30fps), so
+    # marks won't hit 5.350 exactly -- check within one frame, not bit-exact.
+    assert abs(win.offset.delay_ms - (-350.0)) < 34
+    assert win.offset.target == "video"
+
+    win.grab().save(str(OUT_DIR / "ui_smoke_after_marking.png"))
+
+    # Drive export directly through the worker (skip the file-save dialog,
+    # which needs a real user) but through the same ExportWorker class the
+    # button uses.
+    done = {"ok": False, "err": None}
+
+    def on_ok(result):
+        done["ok"] = True
+        print("export finished OK, ffmpeg log:", result.stderr[:200])
+
+    def on_err(msg, stderr):
+        done["err"] = (msg, stderr)
+
+    worker = app_main.ExportWorker(win.input_path, win.offset.delay_ms, EXPORT_OUT)
+    worker.finished_ok.connect(on_ok)
+    worker.finished_err.connect(on_err)
+    worker.start()
+
+    deadline = time.time() + 15
+    while not done["ok"] and done["err"] is None and time.time() < deadline:
+        qapp.processEvents()
+        time.sleep(0.05)
+
+    if done["err"]:
+        raise RuntimeError(f"export failed: {done['err']}")
+    assert done["ok"], "export did not finish within timeout"
+    assert Path(EXPORT_OUT).exists()
+    print(f"exported file exists: {EXPORT_OUT}, size={Path(EXPORT_OUT).stat().st_size} bytes")
+
+    win.player.shutdown()
+    print("\nUI SMOKE TEST PASSED")
+
+
+if __name__ == "__main__":
+    main()