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/Stats.tsx

@@ -7,6 +7,7 @@ import { formatEta, quotaActionLabel } from "../lib/format";
 import { notify } from "../lib/notifications";
 import { LS, useAccountPersistedState } from "../lib/storage";
 import Tooltip from "./Tooltip";
+import { StateMessage } from "./QueryState";
 import { Section, SettingRow } from "./ui/form";
 import { useMe } from "../lib/useMe";
 
@@ -114,6 +115,10 @@ function Overview({ me }: { me: Me }) {
               {s.quota_remaining_today.toLocaleString()}
             </SettingRow>
           </div>
+        ) : status.isError ? (
+          <StateMessage tone="error" onRetry={() => status.refetch()}>
+            {t("common.loadError")}
+          </StateMessage>
         ) : (
           <div className="text-muted text-sm">{t("stats.sync.loading")}</div>
         )}