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

frontend/app.js

@@ -944,7 +944,24 @@ function deletePlaylist(pl) {
   ]);
 }
 
-// ---------- Modal ----------
+// ============================================================================
+// Keyboard shortcut help overlay
+// ============================================================================
+function toggleShortcutHelp() {
+  const overlay = $('shortcutHelp');
+  overlay.classList.toggle('hidden');
+}
+
+function wireShortcutHelp() {
+  $('shortcutClose').addEventListener('click', () => $('shortcutHelp').classList.add('hidden'));
+  $('shortcutHelp').addEventListener('click', (e) => {
+    if (e.target.id === 'shortcutHelp') $('shortcutHelp').classList.add('hidden');
+  });
+}
+
+// ============================================================================
+// Modal
+// ============================================================================
 function showModal(title, bodyNode, actions) {
   $('modalTitle').textContent = title;
   const body = $('modalBody');
@@ -1087,6 +1104,8 @@ function wireUI() {
     else if (e.code === 'ArrowLeft') Player.seek(Math.max(0, Player.master.currentTime - 5));
     else if (e.key === 'f') els.fsBtn.click();
     else if (e.key === 'm') els.muteBtn.click();
+    else if (e.key === '?') toggleShortcutHelp();
+    else if (e.key === 'Escape' && !$('shortcutHelp').classList.contains('hidden')) { $('shortcutHelp').classList.add('hidden'); }
   });
 
   $('modal').addEventListener('click', (e) => { if (e.target.id === 'modal') closeModal(); });
@@ -1098,6 +1117,7 @@ function wireUI() {
 async function boot() {
   wirePlayerEvents();
   wireUI();
+  wireShortcutHelp();
   try {
     const loaded = await API.loadData();
     if (loaded && typeof loaded === 'object') {