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/lib/messagesSocket.ts

@@ -11,8 +11,10 @@ interface IncomingMessage {
   to?: MessageUser;
 }
 type Handler = (e: IncomingMessage) => void;
+type UnreadHandler = () => void;
 
 const handlers = new Set<Handler>();
+const unreadHandlers = new Set<UnreadHandler>();
 let ws: WebSocket | null = null;
 let started = false;
 let backoff = 1000;
@@ -21,7 +23,9 @@ let reconnectTimer: ReturnType<typeof setTimeout> | null = null;
 function url(): string {
   const proto = location.protocol === "https:" ? "wss" : "ws";
   // A WebSocket can't carry the X-Siftlode-Account header, so the per-tab account rides in the
-  // query string (validated against the browser wallet server-side).
+  // query string (validated against the browser wallet server-side). The account is read once
+  // here: switching accounts always goes through location.reload() (NavSidebar), which rebuilds
+  // this module and reopens the socket, so it's intentionally fixed for the socket's lifetime.
   const account = getActiveAccount();
   const qs = account != null ? `?account=${account}` : "";
   return `${proto}://${location.host}/api/messages/ws${qs}`;
@@ -43,6 +47,9 @@ function open() {
       if (data?.type === "message" && data.message) {
         const e: IncomingMessage = { message: data.message as Message, from: data.from, to: data.to };
         for (const h of handlers) h(e);
+      } else if (data?.type === "unread") {
+        // Sent to a user's OTHER tabs when they read a thread elsewhere, so the badge updates live.
+        for (const h of unreadHandlers) h();
       }
     } catch {
       /* ignore malformed frames */
@@ -87,3 +94,13 @@ export function onMessage(h: Handler): () => void {
     }
   };
 }
+
+// Subscribe to "your unread changed elsewhere" pings (a read on another tab/device). The socket is
+// already opened/closed by onMessage subscribers (the app-wide ChatDock keeps one), so this only
+// registers a callback — it does not open the socket on its own.
+export function onUnread(h: UnreadHandler): () => void {
+  unreadHandlers.add(h);
+  return () => {
+    unreadHandlers.delete(h);
+  };
+}