UI/UX polish: Tools menu, clickable timeline, live drag-seek, drag-and-drop load

- Clap detection moved from inline buttons to a Tools menu, plain labels
  (no more '(whole file)' / '(from cursor)' parentheticals) -- status
  messages moved to the window's status bar since the dedicated label
  widget went away with the button row.
- ClickableSlider subclass: clicking anywhere along the seek track jumps
  straight there instead of QSlider's default page-by-a-step behavior;
  reposition-then-delegate-to-super keeps native drag behavior intact.
- Dragging now seeks live via sliderMoved instead of waiting for
  sliderReleased.
- PlayerWidget accepts drag-and-drop of a local file to load it.

Verified with real Qt events (QMouseEvent clicks, a QDropEvent, a real
mouse click via xdotool opening the Tools menu on an actual running
window) rather than just calling the underlying methods directly.

scripts/smoke_test_clap_detection.py

@@ -39,12 +39,12 @@ def main():
     time.sleep(0.3)
     qapp.processEvents()
 
-    assert win.detect_all_button.isEnabled()
-    win.detect_all_button.click()
+    assert win.detect_all_action.isEnabled()
+    win.detect_all_action.trigger()
     qapp.processEvents()
-    assert not win.detect_all_button.isEnabled(), "button should disable while detection runs"
+    assert not win.detect_all_action.isEnabled(), "action should disable while detection runs"
 
-    ok = wait_for(lambda: win.detect_all_button.isEnabled(), qapp)
+    ok = wait_for(lambda: win.detect_all_action.isEnabled(), qapp)
     assert ok, "detection did not finish in time"
 
     markers = win.marker_strip.markers()