feat: add split at cursor functionality

Implemented track splitting at playback cursor:
- Added handleSplitAtCursor function to split selected track at current time
- Extracts two audio segments: before and after cursor position
- Creates two new tracks from the segments with numbered names
- Removes original track after split
- Added 'Split at Cursor' command to command palette (keyboard shortcut: S)
- Validation for edge cases (no track selected, no audio, invalid position)
- User feedback via toast notifications

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

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

components/editor/AudioEditor.tsx

@@ -803,6 +803,66 @@ export function AudioEditor() {
     });
   }, [tracks, executeCommand, updateTrack, addToast]);
 
+  const handleSplitAtCursor = React.useCallback(() => {
+    if (!selectedTrackId) {
+      addToast({
+        title: 'No Track Selected',
+        description: 'Select a track to split',
+        variant: 'error',
+        duration: 2000,
+      });
+      return;
+    }
+
+    const track = tracks.find((t) => t.id === selectedTrackId);
+    if (!track || !track.audioBuffer) {
+      addToast({
+        title: 'No Audio',
+        description: 'Selected track has no audio to split',
+        variant: 'error',
+        duration: 2000,
+      });
+      return;
+    }
+
+    const splitTime = currentTime;
+    const duration = track.audioBuffer.duration;
+
+    // Can't split at the very beginning or end
+    if (splitTime <= 0 || splitTime >= duration) {
+      addToast({
+        title: 'Invalid Split Position',
+        description: 'Cannot split at the beginning or end of the track',
+        variant: 'error',
+        duration: 2000,
+      });
+      return;
+    }
+
+    // Extract two segments
+    const firstSegment = extractBufferSegment(track.audioBuffer, 0, splitTime);
+    const secondSegment = extractBufferSegment(track.audioBuffer, splitTime, duration);
+
+    // Create two new tracks
+    const trackIndex = tracks.indexOf(track);
+    const firstName = `${track.name} (1)`;
+    const secondName = `${track.name} (2)`;
+
+    // Add the two new tracks
+    addTrackFromBuffer(firstSegment, firstName);
+    addTrackFromBuffer(secondSegment, secondName);
+
+    // Remove the original track
+    removeTrack(track.id);
+
+    addToast({
+      title: 'Track Split',
+      description: `Split track at ${formatDuration(splitTime)}`,
+      variant: 'success',
+      duration: 2000,
+    });
+  }, [selectedTrackId, tracks, currentTime, addTrackFromBuffer, removeTrack, addToast]);
+
   // Export handler
   const handleExport = React.useCallback(async (settings: ExportSettings) => {
     if (tracks.length === 0) {
@@ -1381,6 +1441,14 @@ export function AudioEditor() {
         category: 'edit',
         action: handleDuplicate,
       },
+      {
+        id: 'split-at-cursor',
+        label: 'Split at Cursor',
+        description: 'Split selected track at current playback position',
+        shortcut: 'S',
+        category: 'edit',
+        action: handleSplitAtCursor,
+      },
       {
         id: 'select-all',
         label: 'Select All',