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_preview.py

@@ -38,7 +38,8 @@ def main():
     assert win.video_tabs.count() == 2
     assert win.video_tabs.tabText(0) == "Source"
     assert win.video_tabs.tabText(1) == "Preview"
-    assert win.video_tabs.widget(0) is win.player
+    assert win.video_tabs.widget(0) is win.source_panel
+    assert win.video_tabs.widget(1) is win.preview_panel
     print("PASS: video area has Source and Preview tabs")
 
     win._load(CLIP)
@@ -47,11 +48,11 @@ def main():
     qapp.processEvents()
     assert win.video_tabs.currentIndex() == 0, "loading a new source should land on the Source tab"
 
-    win.player.seek(5.000)
+    win.source_panel.player.seek(5.000)
     qapp.processEvents()
     win._mark_video()
 
-    win.player.seek(5.350)
+    win.source_panel.player.seek(5.350)
     qapp.processEvents()
     win._mark_audio()
 
@@ -77,21 +78,41 @@ def main():
     time.sleep(0.3)
     qapp.processEvents()
 
-    duration = win.preview_player.mpv.duration
+    duration = win.preview_panel.player.mpv.duration
     print(f"preview clip duration: {duration:.3f}s (expected ~{win.PREVIEW_DURATION_SEC}s, clamped to file bounds)")
     assert duration is not None and 0 < duration <= win.PREVIEW_DURATION_SEC + 0.5
-    assert win.preview_player.mpv.pause is False, "preview should auto-play"
-    assert win.preview_play_button.text() == "Pause"
+    assert win.preview_panel.player.mpv.pause is False, "preview should auto-play"
+    assert win.preview_panel.play_button.text() == "Pause"
+
+    # -- The Preview tab has its own functional timeline (not a static video
+    # + a single play button): its own seek slider, sized to the preview
+    # clip's own duration, that seeks the preview player independently of
+    # the Source tab's slider/player.
+    assert win.preview_panel.seek_slider is not win.source_panel.seek_slider
+    preview_duration_ms = win.preview_panel.seek_slider.maximum()
+    assert preview_duration_ms > 0
+    print(f"Preview tab's own seek slider range: 0..{preview_duration_ms}ms")
+    target_ms = int(preview_duration_ms * 0.7)
+    win.preview_panel.seek_slider.setValue(target_ms)
+    win.preview_panel.seek_slider.sliderMoved.emit(target_ms)
+    qapp.processEvents()
+    time.sleep(0.2)
+    qapp.processEvents()
+    preview_pos = win.preview_panel.player.get_position()
+    source_pos = win.source_panel.player.get_position()
+    print(f"dragged Preview tab's own slider to 70%: preview player now at {preview_pos:.3f}s, source player still at {source_pos:.3f}s")
+    assert abs(preview_pos - target_ms / 1000.0) < 0.5, "Preview tab's slider should seek the preview player"
+    print("PASS: Preview tab has its own independent, functional timeline")
 
     # -- Switching back to Source shouldn't disturb the main player, and the
     # preview player keeps running independently (two live mpv instances).
-    main_pos_before_switch = win.player.get_position()
+    main_pos_before_switch = win.source_panel.player.get_position()
     win.video_tabs.setCurrentIndex(0)
     qapp.processEvents()
-    main_pos_after_switch = win.player.get_position()
+    main_pos_after_switch = win.source_panel.player.get_position()
     print(f"main player position: before switch={main_pos_before_switch:.3f}s after={main_pos_after_switch:.3f}s")
     assert abs(main_pos_after_switch - main_pos_before_switch) < 0.1, "switching tabs shouldn't move the main player"
-    assert win.player.mpv.pause is True, "main player should still be paused (untouched by preview building)"
+    assert win.source_panel.player.mpv.pause is True, "main player should still be paused (untouched by preview building)"
     print("PASS: switching tabs doesn't disturb the main player")
 
     # -- Building a second preview swaps the file without leaking the first