Polish transport spacing, drop shortcut hints, add Detect Next Clap button

- PlayerPanel now leaves an 8px bottom margin so the transport row
  isn't flush against the tab's edge.
- Frame-step buttons read '⏮ Frame' / 'Frame ⏭' instead of
  '⏮ Frame (,)' / 'Frame ⏭ (.)' -- the shortcuts still work, just not
  spelled out on the buttons.
- Detect Next Clap is now a button in the Sync markers panel next to
  Detect All Claps (still also in the Tools menu), kept in sync with
  its menu action's enabled state the same way Detect All Claps was.

scripts/smoke_test_interactions.py

@@ -30,17 +30,19 @@ def main():
     win.show()
     qapp.processEvents()
 
-    # -- Tools menu has the clap detection actions, AND Detect All Claps is
-    # also back as a button in the Sync markers panel (both should reflect
-    # the same enabled state, kept in sync explicitly since they're two
+    # -- Tools menu has the clap detection actions, AND both Detect All Claps
+    # and Detect Next Clap are also buttons in the Sync markers panel (each
+    # kept in sync with its menu action's enabled state, since they're two
     # separate widgets).
     menu_titles = [a.text() for a in win.menuBar().actions()]
     assert "&Tools" in menu_titles, f"expected a Tools menu, got {menu_titles}"
     assert win.detect_all_action.text() == "Detect All Claps"
     assert win.detect_next_action.text() == "Detect Next Clap"
     assert win.detect_all_button.text() == "Detect All Claps"
+    assert win.detect_next_button.text() == "Detect Next Clap"
     assert win.detect_all_action.isEnabled() == win.detect_all_button.isEnabled()
-    print("PASS: Tools menu has plain-labeled clap detection actions, button back in sync markers panel")
+    assert win.detect_next_action.isEnabled() == win.detect_next_button.isEnabled()
+    print("PASS: Tools menu has plain-labeled clap detection actions, both buttons back in sync markers panel")
 
     # -- Load via drag-and-drop simulation on the player widget
     mime = QMimeData()