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/Scheduler.tsx

@@ -18,6 +18,7 @@ import {
 } from "lucide-react";
 import { api, type SchedulerJob, type SchedulerStatus } from "../lib/api";
 import { useLiveQuery } from "../lib/useLiveQuery";
+import { StateMessage } from "./QueryState";
 import { useConfirm } from "./ConfirmProvider";
 import { relativeTime } from "../lib/format";
 import { notify } from "../lib/notifications";
@@ -422,6 +423,12 @@ export default function Scheduler() {
     },
   });
 
+  if (q.isError && !data)
+    return (
+      <StateMessage tone="error" onRetry={() => q.refetch()}>
+        {t("common.loadError")}
+      </StateMessage>
+    );
   if (q.isLoading && !data) return <div className="p-8 text-muted">{t("scheduler.loading")}</div>;
   if (!data) return <div className="p-8 text-muted">{t("common.somethingWrong")}</div>;