Implement keyboard shortcuts for common editing actions

[clencyc]

Description

Add keyboard shortcut support for faster editing workflow:

• Space: Play/Pause
• I: Mark in point
• O: Mark out point
• Delete: Remove selected clip
• Ctrl/Cmd + Z: Undo
• Ctrl/Cmd + Y: Redo
• Ctrl/Cmd + S: Save project

Acceptance Criteria

• Keyboard event handler implemented in main App component
• All shortcuts listed in help menu
• Shortcuts work only when not typing in input fields
• Mac (Cmd) and Windows (Ctrl) variants handled
• Unit tests for shortcut detection
• Help modal/documentation updated with shortcut list

Implementation Details

• Create hooks/useKeyboardShortcuts.ts custom hook
• Add shortcut configuration object in config/shortcuts.ts
• Integrate with existing chat interface without conflicts
• Update Settings/Help component with shortcut reference

Difficulty: Medium

Estimated time: 2-3 hours