feat: implement multi-track waveform selection and editing with undo/redo

Added comprehensive selection and editing capabilities to multi-track editor:
- Visual selection overlay with Shift+drag interaction on waveforms
- Multi-track edit commands (cut, copy, paste, delete, duplicate)
- Full keyboard shortcut support (Ctrl+X/C/V/D, Delete, Ctrl+Z/Y)
- Complete undo/redo integration via command pattern
- Per-track selection state with localStorage persistence
- Audio buffer manipulation utilities (extract, insert, delete, duplicate segments)
- Toast notifications for all edit operations
- Red playhead to distinguish from blue selection overlay

All edit operations are fully undoable and integrated with the existing
history manager system.

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

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

lib/audio/buffer-utils.ts

@@ -165,3 +165,15 @@ export function concatenateBuffers(
 
   return newBuffer;
 }
+
+/**
+ * Duplicate a segment of audio buffer (extract and insert it after the selection)
+ */
+export function duplicateBufferSegment(
+  buffer: AudioBuffer,
+  startTime: number,
+  endTime: number
+): AudioBuffer {
+  const segment = extractBufferSegment(buffer, startTime, endTime);
+  return insertBufferSegment(buffer, segment, endTime);
+}