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/ChannelDiscovery.tsx

@@ -15,6 +15,7 @@ import ChannelLink from "./ChannelLink";
 import AboutCell from "./AboutCell";
 import DataTable, { type Column } from "./DataTable";
 import ChannelLayoutGrid from "./ChannelLayoutGrid";
+import { StateMessage } from "./QueryState";
 import ColumnSortControl from "./ColumnSortControl";
 import Pager from "./Pager";
 import { PageToolbar } from "./PageShell";
@@ -223,7 +224,11 @@ export default function ChannelDiscovery({
         </div>
       </PageToolbar>
       <div className="px-4 pb-4 max-w-[96rem] mx-auto">
-        {query.isLoading ? (
+        {query.isError ? (
+          <StateMessage tone="error" onRetry={() => query.refetch()}>
+            {t("common.loadError")}
+          </StateMessage>
+        ) : query.isLoading ? (
           <div className="text-muted py-8">{t("channels.discovery.loading")}</div>
         ) : layout === "table" ? (
           <DataTable