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_preserve_play_state.py

@@ -1,14 +1,14 @@
 """Verifies arrow-key skips preserve whichever play/pause state the user
 was already in, instead of always forcing a pause.
 
-Calls the real MainWindow._seek_relative() method directly rather than
+Calls the real PlayerPanel.seek_relative() method directly rather than
 simulating the QShortcut keypress: QTest.keyClick against this shortcut was
 observed to be unreliable in this headless setup even with Qt's
 active-window state forced (see smoke_test_arrow_seek.py's note) --
 sometimes the simulated key silently doesn't reach the shortcut at all, and
 a debug script isolated that PlayerWidget.seek() itself works correctly
 while playing, so the flakiness is in the synthetic-keypress dispatch, not
-the app. The QShortcut -> _seek_relative wiring itself (a single line) was
+the app. The QShortcut -> seek_relative wiring itself (a single line) was
 already independently confirmed with real xdotool keypresses against a
 live window in an earlier session.
 """
@@ -37,37 +37,38 @@ def main():
     qapp.processEvents()
 
     # -- Case 1: paused before the skip -> stays paused after
-    assert win.player.mpv.pause is True
-    before_pos = win.player.get_position()
-    win._seek_relative(5.0)
+    assert win.source_panel.player.mpv.pause is True
+    before_pos = win.source_panel.player.get_position()
+    win.source_panel.seek_relative(5.0)
     qapp.processEvents()
     time.sleep(0.2)
     qapp.processEvents()
-    after_pos = win.player.get_position()
+    after_pos = win.source_panel.player.get_position()
     print(f"paused case: before={before_pos:.3f}s after={after_pos:.3f}s")
-    assert win.player.mpv.pause is True, "should still be paused after skip"
-    assert win.play_button.text() == "Play"
+    assert win.source_panel.player.mpv.pause is True, "should still be paused after skip"
+    assert win.source_panel.play_button.text() == "Play"
     assert after_pos > before_pos + 4.0, "should have actually skipped forward"
     print("PASS: stays paused across arrow-key skip when it started paused")
 
     # -- Case 2: playing before the skip -> stays playing after
-    win.player.seek(0.0)
-    win.player.play()
-    win.play_button.setText("Pause")
+    win.source_panel.player.seek(0.0)
+    win.source_panel.player.play()
+    win.source_panel.play_button.setText("Pause")
     qapp.processEvents()
     time.sleep(0.2)
-    before_pos = win.player.get_position()
-    win._seek_relative(5.0)
+    before_pos = win.source_panel.player.get_position()
+    win.source_panel.seek_relative(5.0)
     qapp.processEvents()
     time.sleep(0.2)
     qapp.processEvents()
-    assert win.player.mpv.pause is False, "should still be playing after skip"
-    assert win.play_button.text() == "Pause"
-    after_pos = win.player.get_position()
+    assert win.source_panel.player.mpv.pause is False, "should still be playing after skip"
+    assert win.source_panel.play_button.text() == "Pause"
+    after_pos = win.source_panel.player.get_position()
     print(f"playing case: before={before_pos:.3f}s after={after_pos:.3f}s (should have jumped ~5s and kept playing)")
     assert after_pos > before_pos + 4.0, "should have actually skipped forward"
 
-    win.player.shutdown()
+    win.source_panel.shutdown()
+    win.preview_panel.shutdown()
     print("\nPRESERVE PLAY STATE SMOKE TEST PASSED")