feat(ui): R3 S1 — honest error states across the pages

The error state didn't exist as a concept: a failed main query rendered as an "empty" list
("No channels" while the request actually died), so a backend restart read as "everything is
empty". Add the convention that an empty state may NEVER show while a query is erroring.

- New components/QueryState.tsx: shared StateMessage (loading / error+Retry, house style, with
  role=status/aria-live — the app had zero live regions).
- DataTable gains loading/error/onRetry props (suppresses empty-text on error).
- isError branch (before the empty branch) added across the false-empty pages: Feed (retry),
  Channels, ChannelDiscovery, AuditLog, NotificationsPanel, Messages (conversations + people),
  Scheduler, Stats, PlexBrowse grid, PlexPlaylistView. ChannelPage's main content is Feed, now
  covered. common.loadError/empty strings (en+hu).

frontend/src/components/PlexPlaylistView.tsx

@@ -33,6 +33,7 @@ import {
 import { api, type PlexCard } from "../lib/api";
 import { getAccountRaw, LS, setAccountRaw } from "../lib/storage";
 import { useConfirm } from "./ConfirmProvider";
+import { StateMessage } from "./QueryState";
 
 // A playlist's items, grouped for readability: movies stay standalone, while a run of episodes from the
 // same show collapses into a season/show group so a whole series doesn't sprawl into an endless flat
@@ -576,7 +577,11 @@ export default function PlexPlaylistView({
           </button>
         </div>
 
-        {q.isLoading ? (
+        {q.isError ? (
+          <StateMessage tone="error" onRetry={() => q.refetch()}>
+            {t("common.loadError")}
+          </StateMessage>
+        ) : q.isLoading ? (
           <p className="py-6 text-center text-sm text-muted">{t("plex.loading")}</p>
         ) : items.length === 0 ? (
           <p className="py-6 text-center text-sm text-muted">{t("plex.playlist.empty")}</p>