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/Feed.tsx

@@ -36,6 +36,7 @@ import {
 import { FEED_VIEWS, type FeedView } from "../lib/feedView";
 import { clearedFilters, useActiveFilters } from "../lib/useActiveFilters";
 import ActiveFilterChips from "./ActiveFilterChips";
+import { StateMessage } from "./QueryState";
 import { PageToolbar } from "./PageShell";
 import { useWizard } from "./WizardProvider";
 import ViewSwitcher from "./ViewSwitcher";
@@ -493,7 +494,12 @@ export default function Feed({
   }
 
   if (query.isLoading) return <div className="p-8 text-muted">{t("feed.loading")}</div>;
-  if (query.isError) return <div className="p-8 text-muted">{t("feed.loadError")}</div>;
+  if (query.isError)
+    return (
+      <StateMessage tone="error" onRetry={() => query.refetch()}>
+        {t("feed.loadError")}
+      </StateMessage>
+    );
   // Empty "my" feed with no YouTube access. The shared demo account can never connect
   // YouTube, so it gets a gentle nudge into the shared library instead of the connect
   // wizard; real users get the onboarding prompt. No toolbar (sort/type are meaningless).