The hint text mentions 'ESC 索引' but the keyboard handler only
supports arrow keys, Home/End. This adds the missing overview:
- ESC toggles a 4-column thumbnail grid overlay
- Each slide is cloned and scaled down as a preview
- Current slide gets a highlighted border
- Clicking a thumbnail navigates to that slide
- Arrow/scroll keys are blocked while overview is open
- Uses existing CSS variables (--ink-rgb, --paper-rgb, --mono)
for consistent theming
