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/PlexBrowse.tsx

@@ -53,6 +53,7 @@ import {
 } from "../lib/plexDetailUi";
 import PlexPlaylistAdd, { type PlexAddTarget } from "./PlexPlaylistAdd";
 import PlexCollectionEditor from "./PlexCollectionEditor";
+import { StateMessage } from "./QueryState";
 import { PageToolbar } from "./PageShell";
 import { usePlex, usePlexActions } from "./PlexProvider";
 
@@ -357,7 +358,11 @@ export default function PlexBrowse() {
         </div>
       )}
 
-      {browseQ.isLoading ? (
+      {browseQ.isError ? (
+        <StateMessage tone="error" onRetry={() => browseQ.refetch()}>
+          {t("common.loadError")}
+        </StateMessage>
+      ) : browseQ.isLoading ? (
         <p className="text-muted text-sm">{t("plex.loading")}</p>
       ) : items.length === 0 && episodes.length === 0 ? (
         dq && plexFilterCount(filters) > 0 ? (