fix: playhead not moving in SECTION-TAB with empty tracks

Two fixes:
1. handlePlayPause: add explicit requestAnimationFrame(updatePlayhead)
   after setIsPlaying(true) to ensure RAF starts immediately.
2. handleEditSectionInTab: add setCurrentTime(0) to reset playhead
   to section start. Without this, currentTime may be outside
   the section's timeline range (e.g. from main session playback).

app/static/js/app.jsx

@@ -8525,6 +8525,7 @@ const App = () => {
     }));
     setSessionTabs(prev => [...prev, { id: tabId, name: tabName, sectionId: sectionId, tracks: clonedTracks, color: section.color || null }]);
     setActiveTab(tabId);
+    setCurrentTime(0);
   };
 
   // ── Sub Tab: apply effects & merge back (LOOP_EDITOR_2.md §1.2) ──
@@ -10614,6 +10615,7 @@ const App = () => {
       startAudioTimeRef.current = context.currentTime;
       startTrackPlayback(currentTime);
       setIsPlaying(true);
+      animationFrameIdRef.current = requestAnimationFrame(updatePlayhead);
     }
   };
   handlePlayPauseRef.current = handlePlayPause;