feat: display keyboard shortcuts on toolbar buttons (#788)

- Add shortcut hints to toolbar buttons via tooltip text
- Integrate useKeyboardShortcutsHelp hook for consistent shortcut retrieval
- Timeline toolbar: split, duplicate, delete, snapping, bookmark buttons
- Preview toolbar: play/pause button
- Format: tooltip text appended with '(Key)' or '(Key1 or Key2)'
- Gracefully handle actions without defined shortcuts