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>

types/selection.ts

@@ -0,0 +1,23 @@
+/**
+ * Selection and region types
+ */
+
+export interface Selection {
+  start: number;
+  end: number;
+}
+
+export interface Region {
+  id: string;
+  start: number;
+  end: number;
+  label?: string;
+  color?: string;
+}
+
+export interface ClipboardData {
+  buffer: AudioBuffer;
+  start: number;
+  end: number;
+  duration: number;
+}