Add clap/onset auto-detection feature

Ports scripts/detect_claps.py (a librosa-based onset detector from an
earlier session) into clap_detection.py, shared by both the CLI script
and a new GUI feature: 'Detect All Claps' (whole-file scan) and
'Detect Next Clap' (from cursor, first result only), with results
shown as clickable markers on a new MarkerStrip widget above the seek
slider -- clicking one seeks the player to that exact timestamp.

Caught a real correctness bug while testing against the out-of-sync
fixture (not just the plain in-sync one): ffmpeg's default -vn audio
extraction silently drops the audio stream's own start_time and
renormalizes to t=0 -- which is exactly the offset this whole tool
exists to fix, so losing it meant detected timestamps pointed at the
wrong track's timing entirely. Fixed by probing the true start_time
via ffprobe and re-inserting it as explicit silence (adelay) before
analysis. Also dropped an earlier attempt at an ffmpeg -ss seek
optimization for the from-cursor case, since combining it with the
start_time fix meant two independent timestamp-origin corrections
stacked on each other -- correct in theory, too easy to get subtly
wrong in a tool where a silent wrong-timestamp bug is the worst
possible failure mode. Full-file decode + filter in Python is slower
but unambiguous.

Verified with a smoke test that drives the real MainWindow buttons,
confirms the detected marker lands near the fixture's known beep
(t=5.350s, allowing for onset-detection lag), and simulates an actual
QMouseEvent click on the marker to confirm click-to-seek works
end-to-end.

scripts/smoke_test_clap_detection.py

@@ -0,0 +1,86 @@
+"""Drives the real MainWindow's clap-detection buttons (not a copy of the
+logic) against the synthetic fixture with a known beep at t=5.350s, then
+simulates a real mouse click on the MarkerStrip widget to confirm
+click-to-seek actually moves the player -- not just that a signal exists.
+"""
+import sys
+import time
+from pathlib import Path
+
+from PyQt6.QtCore import Qt, QPointF
+from PyQt6.QtGui import QMouseEvent
+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"
+
+
+def wait_for(predicate, qapp, timeout=20):
+    deadline = time.time() + timeout
+    while not predicate() and time.time() < deadline:
+        qapp.processEvents()
+        time.sleep(0.05)
+    return predicate()
+
+
+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()
+
+    assert win.detect_all_button.isEnabled()
+    win.detect_all_button.click()
+    qapp.processEvents()
+    assert not win.detect_all_button.isEnabled(), "button should disable while detection runs"
+
+    ok = wait_for(lambda: win.detect_all_button.isEnabled(), qapp)
+    assert ok, "detection did not finish in time"
+
+    markers = win.marker_strip.markers()
+    print(f"detected markers: {markers}")
+    assert len(markers) >= 1, "expected at least one clap marker"
+    # ground truth beep is at t=5.350s in this fixture; onset detection has
+    # some inherent lag, so allow a generous window rather than exact match.
+    assert any(abs(m - 5.350) < 0.5 for m in markers), f"no marker near the known beep: {markers}"
+
+    win.grab().save(str(OUT_DIR / "clap_markers_on_timeline.png"))
+
+    # Simulate a real mouse click on the marker to prove click-to-seek works
+    # end to end, not just that the signal is wired.
+    target_time = min(markers, key=lambda m: abs(m - 5.350))
+    strip = win.marker_strip
+    strip.resize(win.seek_slider.width(), strip.height())
+    x = strip._x_for_time(target_time)
+    click_pos = QPointF(x, 5)
+
+    before_pos = win.player.get_position()
+    event = QMouseEvent(
+        QMouseEvent.Type.MouseButtonPress, click_pos, click_pos,
+        Qt.MouseButton.LeftButton, Qt.MouseButton.LeftButton, Qt.KeyboardModifier.NoModifier,
+    )
+    strip.mousePressEvent(event)
+    qapp.processEvents()
+    time.sleep(0.2)
+    qapp.processEvents()
+
+    after_pos = win.player.get_position()
+    print(f"clicked marker at x={x:.1f} (t={target_time:.3f}s); player position before={before_pos:.3f}s after={after_pos:.3f}s")
+    assert abs(after_pos - target_time) < 0.1, "clicking the marker should seek the player to that timestamp"
+
+    win.player.shutdown()
+    print("\nCLAP DETECTION SMOKE TEST PASSED")
+
+
+if __name__ == "__main__":
+    main()