Add up-next queue panel

Shows upcoming videos in the autoplay queue below now-playing meta.
Clickable to jump to any video. Hidden when queue has 1 item or empty.
Updates on playFromList, playNext, playPrev. Hidden in empty player state.

Files:
- frontend/index.html: upnext section in player pane
- frontend/app.js: renderUpNext(), wired into playFromList/playNext/playPrev
- frontend/styles.css: .upnext, .upnext-item, .upnext-list styles

frontend/index.html

@@ -138,6 +138,15 @@
                 <button id="addPlaylistBtn" class="np-btn" title="Add to a playlist">＋ Playlist</button>
               </div>
             </div>
+
+            <!-- Up next queue -->
+            <div id="upnext" class="upnext hidden">
+              <div class="upnext-header">
+                <span>Up next</span>
+                <span id="upnextCount" class="upnext-count">0</span>
+              </div>
+              <div id="upnextList" class="upnext-list"></div>
+            </div>
           </section>
 
           <!-- List pane -->