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/NotificationsPanel.tsx

@@ -35,6 +35,7 @@ import {
 import { channelYouTubeUrl, relativeFromMs, relativeTime } from "../lib/format";
 import { LEVEL_STYLE } from "./Toaster";
 import { PageToolbar } from "./PageShell";
+import { StateMessage } from "./QueryState";
 import { useNavigationActions } from "./NavigationProvider";
 import { useChannelsActions } from "./ChannelsProvider";
 import { useFeedFilters, useFeedFiltersActions } from "./FeedFiltersProvider";
@@ -150,7 +151,11 @@ export default function NotificationsPanel() {
       </PageToolbar>
 
       <div className="px-4 pb-4 pt-3 max-w-3xl w-full mx-auto space-y-4">
-        {q.isLoading && !q.data && clientItems.length === 0 ? (
+        {q.isError && !hasAny ? (
+          <StateMessage tone="error" onRetry={() => q.refetch()}>
+            {t("common.loadError")}
+          </StateMessage>
+        ) : q.isLoading && !q.data && clientItems.length === 0 ? (
           <div className="p-8 text-muted text-center">{t("common.loading")}</div>
         ) : !hasAny ? (
           <div className="glass rounded-2xl p-10 text-center text-muted">