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>

components/tracks/TrackList.tsx

@@ -20,6 +20,7 @@ export interface TrackListProps {
   onRemoveTrack: (trackId: string) => void;
   onUpdateTrack: (trackId: string, updates: Partial<TrackType>) => void;
   onSeek?: (time: number) => void;
+  onSelectionChange?: (trackId: string, selection: { start: number; end: number } | null) => void;
 }
 
 export function TrackList({
@@ -34,6 +35,7 @@ export function TrackList({
   onRemoveTrack,
   onUpdateTrack,
   onSeek,
+  onSelectionChange,
 }: TrackListProps) {
   const [importDialogOpen, setImportDialogOpen] = React.useState(false);
 
@@ -144,6 +146,11 @@ export function TrackList({
               };
               onUpdateTrack(track.id, { effectChain: updatedChain });
             }}
+            onSelectionChange={
+              onSelectionChange
+                ? (selection) => onSelectionChange(track.id, selection)
+                : undefined
+            }
           />
         ))}
       </div>