Add persistent mini now-playing bar

Fixed bar at bottom of window when video is playing. Shows title,
channel, progress, play/pause toggle. Click to restore player view.

Files:
- frontend/index.html: mini-bar markup
- frontend/app.js: showMiniBar/hideMiniBar/updateMiniBar, wiring
- frontend/styles.css: .mini-bar, .mini-bar-inner, .mini-progress styles

frontend/index.html

@@ -173,6 +173,20 @@
 
     <div id="toastContainer" class="toast-container"></div>
 
+    <!-- Mini now-playing bar -->
+    <div id="miniBar" class="mini-bar hidden">
+      <div class="mini-bar-inner">
+        <button id="miniPlayBtn" class="mini-btn">▶</button>
+        <div class="mini-info">
+          <div id="miniTitle" class="mini-title"></div>
+          <div id="miniChannel" class="mini-channel"></div>
+        </div>
+        <span id="miniTime" class="mini-time">0:00</span>
+        <button id="miniCloseBtn" class="mini-btn mini-close">✕</button>
+      </div>
+      <div id="miniProgress" class="mini-progress"><div id="miniProgressFill" class="mini-progress-fill"></div></div>
+    </div>
+
     <!-- Keyboard shortcuts help overlay -->
     <div id="shortcutHelp" class="shortcut-overlay hidden">
       <div class="shortcut-panel">