fix: enforce Spotify rate limit cooldown server- and client-side

Server tracks _spRateLimitUntil and rejects retries instantly without
hitting Spotify, returning the remaining wait time. Retrying before the
window expired was resetting Spotify's timer (19s → 7s → 48s).

Frontend now shows a live countdown with a note not to retry early.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/spotify.js

@@ -63,13 +63,7 @@ export async function resolveSpotifyLink() {
     }
     if (e.message.startsWith('RATE_LIMITED:')) {
       const secs = parseInt(e.message.split(':')[1], 10) || 30;
-      const mins = secs >= 60 ? `${Math.ceil(secs / 60)} Min.` : `${secs}s`;
-      inner.innerHTML = `<div style="color:var(--text);padding:16px;background:var(--surface);border-radius:10px;border-left:3px solid var(--warning,#f59e0b)">
-        <div style="font-weight:600;margin-bottom:6px">⏳ Spotify Rate Limit</div>
-        <div style="font-size:13px;line-height:1.5;color:var(--text2)">
-          Spotify hat zu viele Anfragen erhalten. Bitte in ca. <strong>${mins}</strong> erneut versuchen.
-        </div>
-      </div>`;
+      _startRateLimitCountdown(inner, secs);
       return;
     }
     if (e.message === 'EDITORIAL_PLAYLIST') {
@@ -232,6 +226,26 @@ async function _loadSpotifyTrackArtwork(tracks, listEl) {
   }
 }
 
+function _startRateLimitCountdown(inner, totalSecs) {
+  const endMs = Date.now() + totalSecs * 1000;
+  const render = (remaining) => {
+    const s = Math.max(0, Math.ceil(remaining / 1000));
+    const label = s >= 60 ? `${Math.ceil(s / 60)} Min.` : `${s}s`;
+    inner.innerHTML = `<div style="color:var(--text);padding:16px;background:var(--surface);border-radius:10px;border-left:3px solid var(--warning,#f59e0b)">
+      <div style="font-weight:600;margin-bottom:6px">⏳ Spotify Rate Limit</div>
+      <div style="font-size:13px;line-height:1.5;color:var(--text2)">
+        Bitte warte noch <strong id="sp-rl-countdown">${label}</strong> — nicht erneut versuchen, das setzt den Timer zurück.
+      </div>
+    </div>`;
+  };
+  render(totalSecs * 1000);
+  const iv = setInterval(() => {
+    const remaining = endMs - Date.now();
+    if (remaining <= 0) { clearInterval(iv); render(0); return; }
+    render(remaining);
+  }, 1000);
+}
+
 function _setArt(el, url) {
   if (!el) return;
   const img = document.createElement('img');