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/styles.css

@@ -517,7 +517,62 @@ input[type="range"]::-webkit-slider-thumb:hover { transform: scale(1.25); }
 }
 
 .player-pane.empty .controls,
-.player-pane.empty .now-meta { display: none; }
+.player-pane.empty .now-meta,
+.player-pane.empty .upnext { display: none; }
+
+/* ===================== Up next queue ===================== */
+.upnext {
+  margin-top: 16px;
+  background: linear-gradient(180deg, var(--bg-2), var(--bg-1));
+  border: 1px solid var(--line);
+  border-radius: var(--radius);
+  padding: 14px 16px;
+}
+.upnext-header {
+  display: flex; align-items: center; gap: 8px;
+  font-family: var(--mono);
+  font-size: 10px; font-weight: 700;
+  text-transform: uppercase; letter-spacing: 0.16em;
+  color: var(--text-dim);
+  margin-bottom: 10px;
+}
+.upnext-count {
+  font-family: var(--mono);
+  font-size: 10px;
+  color: var(--accent);
+  background: rgba(255,75,50,0.12);
+  padding: 0 7px;
+  border-radius: 10px;
+  line-height: 17px;
+}
+.upnext-list {
+  display: flex; flex-direction: column; gap: 6px;
+  max-height: 200px; overflow-y: auto;
+}
+.upnext-item {
+  display: flex; gap: 10px;
+  padding: 6px;
+  border-radius: 9px;
+  cursor: pointer;
+  transition: background 0.16s;
+}
+.upnext-item:hover { background: var(--bg-3); }
+.upnext-item img {
+  width: 80px; aspect-ratio: 16/9;
+  border-radius: 6px;
+  object-fit: cover;
+  background: #000;
+  flex-shrink: 0;
+}
+.upnext-info { min-width: 0; display: flex; flex-direction: column; justify-content: center; }
+.upnext-title {
+  font-size: 12.5px; font-weight: 600;
+  display: -webkit-box; -webkit-line-clamp: 2; -webkit-box-orient: vertical; overflow: hidden;
+}
+.upnext-channel {
+  font-size: 11px; color: var(--text-dim);
+  margin-top: 3px;
+}
 
 /* ===================== List pane ===================== */
 .list-pane {