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/i18n/locales/en/settings.json

@@ -124,7 +124,10 @@
     "resetTitle": "Reset demo state?",
     "resetConfirm": "This clears everything the demo account has accumulated — playlists, hidden/watched/saved videos and settings — and re-seeds a few sample playlists. It can't be undone.",
     "resetDone": "Demo reset — {{count}} sample playlist(s) seeded",
-    "resetFailed": "Couldn't reset the demo account"
+    "resetFailed": "Couldn't reset the demo account",
+    "removed": "Removed from the demo whitelist",
+    "removeTitle": "Remove demo access?",
+    "removeConfirm": "Remove {{email}} from the demo whitelist? They'll no longer be able to enter the demo account."
   },
   "invites": {
     "title": "Access requests",
@@ -141,6 +144,9 @@
     "approveHint": "Approve — whitelist this email and email them they're in.",
     "approve": "Approve",
     "denyHint": "Deny this request.",
-    "deny": "Deny"
+    "deny": "Deny",
+    "denied": "Request denied",
+    "denyTitle": "Deny access request?",
+    "denyConfirm": "Deny the access request from {{email}}?"
   }
 }