Give each tab its own full timeline and transport (PlayerPanel refactor)

Extracts a new PlayerPanel widget (video + its own seek slider/marker
strip + its own centered frame-step/play/mute transport row) into
player_panel.py. Both Source and Preview tabs now use one, so
switching tabs switches a complete, independent monitor -- its own
scrubber, its own play state, its own frame-stepping -- not just the
video frame with a single shared play button underneath.

main.py's global keyboard shortcuts (,/./space/left/right) now
dispatch to whichever panel is active via a small _active_panel()
helper, so they follow the same 'whichever monitor you're looking at'
rule as the on-screen controls.

MarkerStrip (clap detection) and the Mark Audio/Video/offset/export
row stay outside the tabs, at the MainWindow level -- marking and
exporting are inherently about the source file, not the preview clip,
so only the timeline+transport moved, not the whole marking workflow.

Updated every existing smoke test for the new win.source_panel.* /
win.preview_panel.* structure (previously flat win.player,
win.seek_slider, etc.), and added a new check to
smoke_test_preview.py confirming the Preview tab's slider is a
genuinely separate, functional widget -- dragging it seeks the preview
player without moving the source player, which is the actual thing
this change was for.

Verified the full suite on the virtual Xvfb display, plus a live
click-through (mark both events, build preview, confirm the Preview
tab shows its own populated timeline/transport, not the source's).

scripts/smoke_test_interactions.py

@@ -50,7 +50,7 @@ def main():
         Qt.MouseButton.LeftButton, Qt.KeyboardModifier.NoModifier,
         QDropEvent.Type.Drop,
     )
-    win.player.dropEvent(drop_event)
+    win.source_panel.player.dropEvent(drop_event)
     qapp.processEvents()
     time.sleep(0.3)
     qapp.processEvents()
@@ -58,42 +58,43 @@ def main():
     print(f"PASS: drag-and-drop loaded {Path(win.input_path).name}")
 
     # -- Clicking directly on the slider track (not the handle) jumps there
-    duration_ms = win.seek_slider.maximum()
+    duration_ms = win.source_panel.seek_slider.maximum()
     assert duration_ms > 0
-    target_x = win.seek_slider.width() * 0.5
-    click_pos = QPointF(target_x, win.seek_slider.height() / 2)
+    target_x = win.source_panel.seek_slider.width() * 0.5
+    click_pos = QPointF(target_x, win.source_panel.seek_slider.height() / 2)
     press_event = QMouseEvent(
         QMouseEvent.Type.MouseButtonPress, click_pos, click_pos,
         Qt.MouseButton.LeftButton, Qt.MouseButton.LeftButton, Qt.KeyboardModifier.NoModifier,
     )
-    before = win.player.get_position()
-    win.seek_slider.mousePressEvent(press_event)
+    before = win.source_panel.player.get_position()
+    win.source_panel.seek_slider.mousePressEvent(press_event)
     qapp.processEvents()
     time.sleep(0.2)
     qapp.processEvents()
-    after = win.player.get_position()
+    after = win.source_panel.player.get_position()
     expected = 0.5 * (duration_ms / 1000.0)
     print(f"clicked slider track at 50%: before={before:.3f}s after={after:.3f}s expected~{expected:.3f}s")
     assert abs(after - expected) < 0.5, "clicking the track should jump the player near that position"
     print("PASS: click-anywhere-on-track seeking works")
 
     # -- Dragging (sliderMoved) seeks live, without waiting for release
-    win.seek_slider.sliderPressed.emit()
+    win.source_panel.seek_slider.sliderPressed.emit()
     qapp.processEvents()
     drag_value = int(0.2 * duration_ms)
-    win.seek_slider.setValue(drag_value)
-    win.seek_slider.sliderMoved.emit(drag_value)
+    win.source_panel.seek_slider.setValue(drag_value)
+    win.source_panel.seek_slider.sliderMoved.emit(drag_value)
     qapp.processEvents()
     time.sleep(0.2)
     qapp.processEvents()
-    live_pos = win.player.get_position()
+    live_pos = win.source_panel.player.get_position()
     expected_drag = drag_value / 1000.0
     print(f"dragged to 20% (still 'pressed', not released): live player position={live_pos:.3f}s expected~{expected_drag:.3f}s")
     assert abs(live_pos - expected_drag) < 0.5, "dragging should seek immediately, before release"
-    win.seek_slider.sliderReleased.emit()
+    win.source_panel.seek_slider.sliderReleased.emit()
     print("PASS: live-seek during drag works")
 
-    win.player.shutdown()
+    win.source_panel.shutdown()
+    win.preview_panel.shutdown()
     print("\nALL INTERACTION SMOKE TESTS PASSED")