Add keyboard-shortcut help overlay

Press '?' to open a styled overlay showing all keyboard shortcuts,
Esc to close. Lists Space (play/pause), arrow keys (seek/volume),
F (fullscreen), M (mute), ? (help), Esc (close).

Files:
- frontend/app.js: toggleShortcutHelp(), wireShortcutHelp(), '?' and Esc handlers
- frontend/index.html: shortcut-overlay markup with .shortcut-grid
- frontend/styles.css: .shortcut-overlay, .shortcut-panel, .shortcut-row, kbd styles

BACKLOG.md

@@ -17,7 +17,7 @@ bottom as they come up.
 ## UX / polish
 - [x] Empty-state designs for empty playlists and empty history (match the new hero).
 - [x] Loading skeletons for search results instead of a bare "Searching…".
-- [ ] Keyboard-shortcut help overlay (press `?`), listing space / arrows / f / m.
+- [x] Keyboard-shortcut help overlay (press `?`), listing space / arrows / f / m.
 - [ ] Download/preload progress indicator on cards (percent or a bar, not just a pulse).
 - [ ] Remember and restore the last playback position per video.
 - [ ] Toast queue (stack multiple toasts instead of replacing).