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>

types/track.ts

@@ -3,6 +3,7 @@
  */
 
 import type { EffectChain } from '@/lib/audio/effects/chain';
+import type { Selection } from './selection';
 
 export interface Track {
   id: string;
@@ -24,6 +25,9 @@ export interface Track {
   // UI state
   collapsed: boolean;
   selected: boolean;
+
+  // Selection (for editing operations)
+  selection: Selection | null;
 }
 
 export interface TrackState {