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/AudioEditor.tsx

@@ -7,7 +7,9 @@ import { Waveform } from './Waveform';
 import { PlaybackControls } from './PlaybackControls';
 import { ZoomControls } from './ZoomControls';
 import { EditControls } from './EditControls';
+import { HistoryControls } from './HistoryControls';
 import { useAudioPlayer } from '@/lib/hooks/useAudioPlayer';
+import { useHistory } from '@/lib/hooks/useHistory';
 import { useToast } from '@/components/ui/Toast';
 import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/Card';
 import { Slider } from '@/components/ui/Slider';
@@ -19,6 +21,12 @@ import {
   insertBufferSegment,
   trimBuffer,
 } from '@/lib/audio/buffer-utils';
+import {
+  createCutCommand,
+  createDeleteCommand,
+  createPasteCommand,
+  createTrimCommand,
+} from '@/lib/history/commands/edit-command';
 
 export function AudioEditor() {
   // Zoom and scroll state
@@ -29,6 +37,7 @@ export function AudioEditor() {
   // Selection state
   const [selection, setSelection] = React.useState<Selection | null>(null);
   const [clipboard, setClipboard] = React.useState<ClipboardData | null>(null);
+
   const {
     loadFile,
     loadBuffer,
@@ -51,6 +60,7 @@ export function AudioEditor() {
     durationFormatted,
   } = useAudioPlayer();
 
+  const { execute, undo, redo, clear: clearHistory, state: historyState } = useHistory(50);
   const { addToast } = useToast();
 
   const handleFileSelect = async (file: File) => {
@@ -79,6 +89,7 @@ export function AudioEditor() {
     setAmplitudeScale(1);
     setSelection(null);
     setClipboard(null);
+    clearHistory();
     addToast({
       title: 'Audio cleared',
       description: 'Audio file has been removed',
@@ -101,9 +112,11 @@ export function AudioEditor() {
         duration: selection.end - selection.start,
       });
 
-      // Delete from buffer
-      const newBuffer = deleteBufferSegment(audioBuffer, selection.start, selection.end);
-      loadBuffer(newBuffer);
+      // Create and execute cut command
+      const command = createCutCommand(audioBuffer, selection, (buffer) => {
+        loadBuffer(buffer);
+      });
+      execute(command);
 
       setSelection(null);
       addToast({
@@ -155,8 +168,12 @@ export function AudioEditor() {
 
     try {
       const insertTime = currentTime;
-      const newBuffer = insertBufferSegment(audioBuffer, clipboard.buffer, insertTime);
-      loadBuffer(newBuffer);
+
+      // Create and execute paste command
+      const command = createPasteCommand(audioBuffer, clipboard.buffer, insertTime, (buffer) => {
+        loadBuffer(buffer);
+      });
+      execute(command);
 
       addToast({
         title: 'Pasted',
@@ -178,8 +195,11 @@ export function AudioEditor() {
     if (!selection || !audioBuffer) return;
 
     try {
-      const newBuffer = deleteBufferSegment(audioBuffer, selection.start, selection.end);
-      loadBuffer(newBuffer);
+      // Create and execute delete command
+      const command = createDeleteCommand(audioBuffer, selection, (buffer) => {
+        loadBuffer(buffer);
+      });
+      execute(command);
 
       setSelection(null);
       addToast({
@@ -202,8 +222,11 @@ export function AudioEditor() {
     if (!selection || !audioBuffer) return;
 
     try {
-      const newBuffer = trimBuffer(audioBuffer, selection.start, selection.end);
-      loadBuffer(newBuffer);
+      // Create and execute trim command
+      const command = createTrimCommand(audioBuffer, selection, (buffer) => {
+        loadBuffer(buffer);
+      });
+      execute(command);
 
       setSelection(null);
       addToast({
@@ -263,6 +286,32 @@ export function AudioEditor() {
         return;
       }
 
+      // Ctrl+Z: Undo
+      if (e.ctrlKey && !e.shiftKey && e.key === 'z') {
+        e.preventDefault();
+        if (undo()) {
+          addToast({
+            title: 'Undo',
+            description: 'Last action undone',
+            variant: 'info',
+            duration: 1500,
+          });
+        }
+      }
+
+      // Ctrl+Y or Ctrl+Shift+Z: Redo
+      if ((e.ctrlKey && e.key === 'y') || (e.ctrlKey && e.shiftKey && e.key === 'z')) {
+        e.preventDefault();
+        if (redo()) {
+          addToast({
+            title: 'Redo',
+            description: 'Last action redone',
+            variant: 'info',
+            duration: 1500,
+          });
+        }
+      }
+
       // Ctrl+A: Select all
       if (e.ctrlKey && e.key === 'a') {
         e.preventDefault();
@@ -302,7 +351,7 @@ export function AudioEditor() {
 
     window.addEventListener('keydown', handleKeyDown);
     return () => window.removeEventListener('keydown', handleKeyDown);
-  }, [selection, clipboard, audioBuffer, currentTime]);
+  }, [selection, clipboard, audioBuffer, currentTime, undo, redo, addToast]);
 
   // Show error toast
   React.useEffect(() => {
@@ -400,6 +449,21 @@ export function AudioEditor() {
             </CardContent>
           </Card>
 
+          {/* History Controls */}
+          <Card>
+            <CardHeader>
+              <CardTitle>History</CardTitle>
+            </CardHeader>
+            <CardContent>
+              <HistoryControls
+                historyState={historyState}
+                onUndo={undo}
+                onRedo={redo}
+                onClear={clearHistory}
+              />
+            </CardContent>
+          </Card>
+
           {/* Zoom Controls */}
           <Card>
             <CardHeader>