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/history/commands/edit-command.ts

@@ -0,0 +1,155 @@
+/**
+ * Edit commands for audio buffer operations
+ */
+
+import { BaseCommand } from '../command';
+import type { Selection } from '@/types/selection';
+import {
+  extractBufferSegment,
+  deleteBufferSegment,
+  insertBufferSegment,
+  trimBuffer,
+} from '@/lib/audio/buffer-utils';
+
+export type EditCommandType = 'cut' | 'delete' | 'paste' | 'trim';
+
+export interface EditCommandParams {
+  type: EditCommandType;
+  beforeBuffer: AudioBuffer;
+  afterBuffer: AudioBuffer;
+  selection?: Selection;
+  clipboardData?: AudioBuffer;
+  pastePosition?: number;
+  onApply: (buffer: AudioBuffer) => void;
+}
+
+/**
+ * Command for edit operations (cut, delete, paste, trim)
+ */
+export class EditCommand extends BaseCommand {
+  private type: EditCommandType;
+  private beforeBuffer: AudioBuffer;
+  private afterBuffer: AudioBuffer;
+  private selection?: Selection;
+  private clipboardData?: AudioBuffer;
+  private pastePosition?: number;
+  private onApply: (buffer: AudioBuffer) => void;
+
+  constructor(params: EditCommandParams) {
+    super();
+    this.type = params.type;
+    this.beforeBuffer = params.beforeBuffer;
+    this.afterBuffer = params.afterBuffer;
+    this.selection = params.selection;
+    this.clipboardData = params.clipboardData;
+    this.pastePosition = params.pastePosition;
+    this.onApply = params.onApply;
+  }
+
+  execute(): void {
+    this.onApply(this.afterBuffer);
+  }
+
+  undo(): void {
+    this.onApply(this.beforeBuffer);
+  }
+
+  getDescription(): string {
+    switch (this.type) {
+      case 'cut':
+        return 'Cut';
+      case 'delete':
+        return 'Delete';
+      case 'paste':
+        return 'Paste';
+      case 'trim':
+        return 'Trim';
+      default:
+        return 'Edit';
+    }
+  }
+
+  /**
+   * Get the type of edit operation
+   */
+  getType(): EditCommandType {
+    return this.type;
+  }
+
+  /**
+   * Get the selection that was affected
+   */
+  getSelection(): Selection | undefined {
+    return this.selection;
+  }
+}
+
+/**
+ * Factory functions to create edit commands
+ */
+
+export function createCutCommand(
+  buffer: AudioBuffer,
+  selection: Selection,
+  onApply: (buffer: AudioBuffer) => void
+): EditCommand {
+  const afterBuffer = deleteBufferSegment(buffer, selection.start, selection.end);
+
+  return new EditCommand({
+    type: 'cut',
+    beforeBuffer: buffer,
+    afterBuffer,
+    selection,
+    onApply,
+  });
+}
+
+export function createDeleteCommand(
+  buffer: AudioBuffer,
+  selection: Selection,
+  onApply: (buffer: AudioBuffer) => void
+): EditCommand {
+  const afterBuffer = deleteBufferSegment(buffer, selection.start, selection.end);
+
+  return new EditCommand({
+    type: 'delete',
+    beforeBuffer: buffer,
+    afterBuffer,
+    selection,
+    onApply,
+  });
+}
+
+export function createPasteCommand(
+  buffer: AudioBuffer,
+  clipboardData: AudioBuffer,
+  pastePosition: number,
+  onApply: (buffer: AudioBuffer) => void
+): EditCommand {
+  const afterBuffer = insertBufferSegment(buffer, clipboardData, pastePosition);
+
+  return new EditCommand({
+    type: 'paste',
+    beforeBuffer: buffer,
+    afterBuffer,
+    clipboardData,
+    pastePosition,
+    onApply,
+  });
+}
+
+export function createTrimCommand(
+  buffer: AudioBuffer,
+  selection: Selection,
+  onApply: (buffer: AudioBuffer) => void
+): EditCommand {
+  const afterBuffer = trimBuffer(buffer, selection.start, selection.end);
+
+  return new EditCommand({
+    type: 'trim',
+    beforeBuffer: buffer,
+    afterBuffer,
+    selection,
+    onApply,
+  });
+}