Implement undo/redo stack for editing operations

[clencyc]

Description

Add full undo/redo functionality so users can revert editing changes without losing progress.

Acceptance Criteria

• Undo/Redo stack maintained in Redux or Context
• Keyboard shortcuts: Ctrl+Z (undo), Ctrl+Y (redo)
• Menu items for undo/redo with keyboard hints
• Undo/Redo buttons in toolbar
• Stack persists within session (not across page reload)
• Max stack size: 50 operations (configurable)
• Works for all editing operations:
  • Clips added/removed
  • Clips moved/trimmed
  • Effects added/removed
  • Audio levels changed
• Unit tests for undo/redo operations

Implementation Details

• Create hooks/useUndoRedo.ts custom hook
• Each operation stored as: { action, data, timestamp }
• undo() pops from undo stack, pushes to redo stack
• redo() pops from redo stack, pushes to undo stack

Difficulty: Hard

Estimated time: 5-6 hours