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/index.html

@@ -164,6 +164,25 @@
 
     <div id="toast" class="toast hidden"></div>
 
+    <!-- Keyboard shortcuts help overlay -->
+    <div id="shortcutHelp" class="shortcut-overlay hidden">
+      <div class="shortcut-panel">
+        <div class="shortcut-header">
+          <h2>Keyboard Shortcuts</h2>
+          <button id="shortcutClose" class="shortcut-close">✕</button>
+        </div>
+        <div class="shortcut-grid">
+          <div class="shortcut-row"><kbd>Space</kbd><span>Play / Pause</span></div>
+          <div class="shortcut-row"><kbd>←</kbd><kbd>→</kbd><span>Seek backward / forward 5s</span></div>
+          <div class="shortcut-row"><kbd>↑</kbd><kbd>↓</kbd><span>Volume up / down</span></div>
+          <div class="shortcut-row"><kbd>F</kbd><span>Toggle fullscreen</span></div>
+          <div class="shortcut-row"><kbd>M</kbd><span>Toggle mute</span></div>
+          <div class="shortcut-row"><kbd>?</kbd><span>Show this help</span></div>
+          <div class="shortcut-row"><kbd>Esc</kbd><span>Close help</span></div>
+        </div>
+      </div>
+    </div>
+
     <script src="app.js"></script>
   </body>
 </html>