feat: implement Phase 5 - undo/redo system with command pattern

Added comprehensive undo/redo functionality:
- Command pattern interface and base classes
- HistoryManager with 50-operation stack
- EditCommand for all edit operations (cut, delete, paste, trim)
- Full keyboard shortcuts (Ctrl+Z undo, Ctrl+Y/Ctrl+Shift+Z redo)
- HistoryControls UI component with visual feedback
- Integrated history system with all edit operations
- Toast notifications for undo/redo actions
- History state tracking and display

New files:
- lib/history/command.ts - Command interface and BaseCommand
- lib/history/history-manager.ts - HistoryManager class
- lib/history/commands/edit-command.ts - EditCommand and factory functions
- lib/hooks/useHistory.ts - React hook for history management
- components/editor/HistoryControls.tsx - History UI component

Modified files:
- components/editor/AudioEditor.tsx - Integrated history system
- components/editor/EditControls.tsx - Updated keyboard shortcuts display

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

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

components/editor/EditControls.tsx

@@ -125,7 +125,7 @@ export function EditControls({
 
       {/* Keyboard Shortcuts Info */}
       <div className="text-xs text-muted-foreground space-y-1 p-3 rounded-lg bg-muted/30">
-        <p className="font-medium mb-2">Keyboard Shortcuts:</p>
+        <p className="font-medium mb-2">Edit Shortcuts:</p>
         <p>• Shift+Drag: Select region</p>
         <p>• Ctrl+A: Select all</p>
         <p>• Ctrl+X: Cut</p>