feat: implement Phase 4 - selection and editing features

Added comprehensive audio editing capabilities:
- Region selection with Shift+drag on waveform
- Visual selection feedback with blue overlay and borders
- AudioBuffer manipulation utilities (cut, copy, paste, delete, trim)
- EditControls UI component with edit buttons
- Keyboard shortcuts (Ctrl+A, Ctrl+X, Ctrl+C, Ctrl+V, Delete, Escape)
- Clipboard management for cut/copy/paste operations
- Updated useAudioPlayer hook with loadBuffer method

New files:
- types/selection.ts - Selection and ClipboardData interfaces
- lib/audio/buffer-utils.ts - AudioBuffer manipulation utilities
- components/editor/EditControls.tsx - Edit controls UI

Modified files:
- components/editor/Waveform.tsx - Added selection support
- components/editor/AudioEditor.tsx - Integrated edit operations
- lib/hooks/useAudioPlayer.ts - Added loadBuffer method

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

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

lib/hooks/useAudioPlayer.ts

@@ -7,6 +7,7 @@ import { decodeAudioFile, formatDuration } from '@/lib/audio/decoder';
 export interface UseAudioPlayerReturn {
   // File management
   loadFile: (file: File) => Promise<void>;
+  loadBuffer: (buffer: AudioBuffer, name?: string) => void;
   clearFile: () => void;
 
   // Playback controls
@@ -99,6 +100,21 @@ export function useAudioPlayer(): UseAudioPlayerReturn {
     [player]
   );
 
+  const loadBuffer = React.useCallback(
+    (buffer: AudioBuffer, name?: string) => {
+      if (!player) return;
+
+      player.loadBuffer(buffer);
+      setAudioBuffer(buffer);
+      if (name) setFileName(name);
+      setDuration(buffer.duration);
+      setCurrentTime(0);
+      setIsPlaying(false);
+      setIsPaused(false);
+    },
+    [player]
+  );
+
   const clearFile = React.useCallback(() => {
     if (!player) return;
 
@@ -173,6 +189,7 @@ export function useAudioPlayer(): UseAudioPlayerReturn {
 
   return {
     loadFile,
+    loadBuffer,
     clearFile,
     play,
     pause,