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>

lib/hooks/useHistory.ts

@@ -0,0 +1,54 @@
+'use client';
+
+import * as React from 'react';
+import { HistoryManager } from '@/lib/history/history-manager';
+import type { HistoryState } from '@/lib/history/history-manager';
+import type { Command } from '@/lib/history/command';
+
+export interface UseHistoryReturn {
+  execute: (command: Command) => void;
+  undo: () => boolean;
+  redo: () => boolean;
+  clear: () => void;
+  state: HistoryState;
+}
+
+export function useHistory(maxHistorySize: number = 50): UseHistoryReturn {
+  const [manager] = React.useState(() => new HistoryManager(maxHistorySize));
+  const [state, setState] = React.useState<HistoryState>(manager.getState());
+
+  React.useEffect(() => {
+    const unsubscribe = manager.subscribe(() => {
+      setState(manager.getState());
+    });
+
+    return unsubscribe;
+  }, [manager]);
+
+  const execute = React.useCallback(
+    (command: Command) => {
+      manager.execute(command);
+    },
+    [manager]
+  );
+
+  const undo = React.useCallback(() => {
+    return manager.undo();
+  }, [manager]);
+
+  const redo = React.useCallback(() => {
+    return manager.redo();
+  }, [manager]);
+
+  const clear = React.useCallback(() => {
+    manager.clear();
+  }, [manager]);
+
+  return {
+    execute,
+    undo,
+    redo,
+    clear,
+    state,
+  };
+}