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/de/downloads.json

@@ -185,5 +185,17 @@
     "added": "Hinzugefügt",
     "user": "Nutzer"
   },
-  "clipBadge": "Clip"
+  "clipBadge": "Clip",
+  "errors": {
+    "quota": {
+      "max_bytes": "Dein Download-Speicherkontingent ist aufgebraucht ({{size}} GB). Gib zuerst Speicher frei.",
+      "max_jobs": "Du hast dein Download-Limit erreicht ({{limit}} Objekte). Entferne zuerst einige.",
+      "generic": "Download-Kontingent überschritten."
+    },
+    "edit": {
+      "empty_edit": "Wähle zuerst einen Zuschneidebereich oder Ausschnitt.",
+      "source_not_ready": "Der Quell-Download ist noch nicht bearbeitbar.",
+      "generic": "Dieser Schnitt konnte nicht erstellt werden."
+    }
+  }
 }