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/command.ts

@@ -0,0 +1,38 @@
+/**
+ * Command Pattern for Undo/Redo System
+ */
+
+export interface Command {
+  /**
+   * Execute the command
+   */
+  execute(): void;
+
+  /**
+   * Undo the command
+   */
+  undo(): void;
+
+  /**
+   * Redo the command (default: call execute again)
+   */
+  redo(): void;
+
+  /**
+   * Get a description of the command for UI display
+   */
+  getDescription(): string;
+}
+
+/**
+ * Base command class with default redo implementation
+ */
+export abstract class BaseCommand implements Command {
+  abstract execute(): void;
+  abstract undo(): void;
+  abstract getDescription(): string;
+
+  redo(): void {
+    this.execute();
+  }
+}