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/Channels.tsx

@@ -28,6 +28,7 @@ import { notify } from "../lib/notifications";
 import Tooltip from "./Tooltip";
 import DataTable, { type Column } from "./DataTable";
 import ChannelLayoutGrid from "./ChannelLayoutGrid";
+import { StateMessage } from "./QueryState";
 import Overlay from "./Overlay";
 import ViewSwitcher, { type ViewOption } from "./ViewSwitcher";
 import ColumnSortControl from "./ColumnSortControl";
@@ -709,7 +710,11 @@ export default function Channels() {
             (status chips + pager) rides in the fixed band right above it (controlsInBand). The
             card layout swaps the table for a virtualized grid over the same rows (its pager and
             status chips ride in the fixed band above instead). */}
-          {channelsQuery.isLoading ? (
+          {channelsQuery.isError ? (
+            <StateMessage tone="error" onRetry={() => channelsQuery.refetch()}>
+              {t("common.loadError")}
+            </StateMessage>
+          ) : channelsQuery.isLoading ? (
             <div className="text-muted py-8">{t("channels.loading")}</div>
           ) : layout === "table" ? (
             <DataTable