feat: add loop playback functionality

Added complete loop functionality with UI controls:
- Loop state management in useMultiTrackPlayer (loopEnabled, loopStart, loopEnd)
- Automatic restart from loop start when reaching loop end during playback
- Loop toggle button in PlaybackControls with Repeat icon
- Loop points UI showing when loop is enabled (similar to punch in/out)
- Manual loop point adjustment with number inputs
- Quick set buttons to set loop points to current time
- Wired loop functionality through AudioEditor component

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

components/editor/AudioEditor.tsx

@@ -294,6 +294,12 @@ export function AudioEditor() {
     stop,
     seek,
     togglePlayPause,
+    loopEnabled,
+    loopStart,
+    loopEnd,
+    toggleLoop,
+    setLoopPoints,
+    setLoopFromSelection,
   } = useMultiTrackPlayer(tracks, masterVolume, handleAutomationRecording);
 
   // Reset latch triggered state when playback stops
@@ -1972,6 +1978,11 @@ export function AudioEditor() {
             onPunchOutTimeChange={setPunchOutTime}
             overdubEnabled={overdubEnabled}
             onOverdubEnabledChange={setOverdubEnabled}
+            loopEnabled={loopEnabled}
+            loopStart={loopStart}
+            loopEnd={loopEnd}
+            onToggleLoop={toggleLoop}
+            onSetLoopPoints={setLoopPoints}
           />
         </div>
       </div>