fix(deferred): close frontend UI/UX tails from the hygiene sweep

- player YB2: keepalive pagehide beacon (api.saveProgressBeacon) so the resume
  position isn't lost on F5/close within 5s of the last checkpoint (mirrors Plex).
- player YB4: bound consecutive unplayable items in auto-advance so an
  auto-advancing/loop=all queue can't spin over a run of dead videos.
- downloads B6 (client): localise structured quota/edit errors centrally in
  api.req() via localizeDetail() + Intl.NumberFormat (fixes HU/DE + decimal sep);
  + 3-locale download error strings.
- channels FB1: focusChannelToken bump so re-clicking the same channel re-seeds
  the search box; FB2: syncSubs invalidates feed/feed-count (set can change now).
- config CB2: reseed the ConfigPanel draft on a key-set dataVersion + explicit
  post-save token, so a mid-edit refetch can't clobber an in-progress edit.
- config AB3 (UI): a blank allow_empty field stores "" instead of resetting.
- admin SB2/SC2: confirm + success toast on demo-whitelist remove and deny-invite
  (+ trilingual strings); SC1: disable only the acted-on row (pendingId), not all;
  SB3: pause the 1s scheduler countdown ticker while the tab is hidden.
- messages CT4: only invalidate conversations/unread when the incoming-message
  count actually changed, not on every 20s poll; MB3 (client): react to the live
  unread ping (onUnread → invalidate); MC3: extract e2ee installKey (setup/unlock);
  CC2: hoist POLL_MS to messaging.ts; MB4: document the reload-scoped socket.

frontend/src/components/PlayerModal.tsx

@@ -107,6 +107,10 @@ export default function PlayerModal({
   const mountRef = useRef<HTMLDivElement | null>(null);
   const playerRef = useRef<any>(null);
   const autoMarkedRef = useRef(false);
+  // Bound consecutive unplayable items so an auto-advancing queue (esp. loop=all) can't spin over a
+  // run of dead videos — after the cap we stop on the error overlay instead of skipping forever.
+  const MAX_CONSECUTIVE_ERRORS = 3;
+  const errorStreakRef = useRef(0);
   // Keyboard/wheel controls. We keep focus on the modal card (not the cross-origin player
   // iframe) so F / Space / Esc keep working until the user clicks into YouTube's native
   // controls; the interaction overlay below also stops the iframe stealing focus on a video
@@ -502,6 +506,7 @@ export default function PlayerModal({
           onStateChange: (e: any) => {
             // 1 === playing → sync the navigated-to video's title/author for display.
             if (e?.data === 1) {
+              errorStreakRef.current = 0; // a successful play breaks any unplayable run
               const p = playerRef.current;
               const d = p && typeof p.getVideoData === "function" ? p.getVideoData() : null;
               if (d) setLiveData({ title: d.title, author: d.author });
@@ -517,17 +522,47 @@ export default function PlayerModal({
           },
           // The embed couldn't play this video (embedding disabled, removed, private…).
           // Surface our own message + an "Open on YouTube" escape hatch.
-          onError: (e: any) => setPlayerError(typeof e?.data === "number" ? e.data : -1),
+          onError: (e: any) => {
+            setPlayerError(typeof e?.data === "number" ? e.data : -1);
+            // The IFrame API fires onError (not 'ended') for an unplayable item, so advanceOnEnd()
+            // never runs and an auto-advancing queue would freeze here. Skip forward instead — but
+            // bounded, so a run of dead items (or loop=all over a broken queue) can't spin. We do
+            // NOT mark the broken video watched (only the ended-handler does that).
+            if (currentIdRef.current !== id) return; // a navigated description-link error: leave the queue alone
+            const { autoMode: a } = modeRef.current;
+            const { queue: q } = navRef.current;
+            if (a === "off" || !q || q.length <= 1) return;
+            errorStreakRef.current += 1;
+            if (errorStreakRef.current >= MAX_CONSECUTIVE_ERRORS) return; // give up; leave the overlay
+            advanceOnEnd();
+          },
         },
       });
     });
 
     // Periodic checkpoint so progress (and auto-watch) survive a crash/refresh.
     const timer = window.setInterval(persist, 5000);
+    // Save on page unload too (F5/close/navigate): effect cleanup doesn't run on a full reload, so
+    // without this the resume point lags up to 5s (a seek right before F5 would be lost). A keepalive
+    // beacon so the POST survives the unload. Only for the library item we opened with — a navigated
+    // description-link video may not be in this user's library (the server would 404).
+    const onHide = () => {
+      const p = playerRef.current;
+      if (!p || typeof p.getCurrentTime !== "function" || currentIdRef.current !== id) return;
+      try {
+        const cur = p.getCurrentTime();
+        const dur = typeof p.getDuration === "function" ? p.getDuration() : 0;
+        if (cur > 0 && dur > 0) api.saveProgressBeacon(id, cur, dur);
+      } catch {
+        /* player tearing down */
+      }
+    };
+    window.addEventListener("pagehide", onHide);
 
     return () => {
       cancelled = true;
       window.clearInterval(timer);
+      window.removeEventListener("pagehide", onHide);
       // Final flush, then refresh the feed + playlists so resume bars reflect this session.
       Promise.resolve(persist()).finally(() => {
         qc.invalidateQueries({ queryKey: ["feed"] });