revert: playhead fixes cause cannot play on MAIN/SECTION-TAB

Revert two changes from 5d31563:
1. Explicit requestAnimationFrame in handlePlayPause (redundant,
   useEffect already handles RAF start after setIsPlaying)
2. setCurrentTime(0) in handleEditSectionInTab (resets global
   currentTime, interferes with MAIN session position)

app/static/js/app.jsx

@@ -8525,7 +8525,6 @@ 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) ──
@@ -10615,7 +10614,6 @@ const App = () => {
       startAudioTimeRef.current = context.currentTime;
       startTrackPlayback(currentTime);
       setIsPlaying(true);
-      animationFrameIdRef.current = requestAnimationFrame(updatePlayhead);
     }
   };
   handlePlayPauseRef.current = handlePlayPause;