fix(ui): R3 S1 review fixes — don't blank data on transient refetch errors

- The honest-state branch now guards on `isError && !data`, so a background/window-focus refetch
  failure keeps already-visible content instead of replacing the list with a full error screen
  (regression the bare `isError` introduced). Applied to AuditLog, Channels, ChannelDiscovery,
  PlexBrowse, PlexPlaylistView, Messages(people), Feed — matching the guard already used in
  Messages(conversations)/NotificationsPanel/Scheduler.
- Drop the unused DataTable loading/error/onRetry props (every table page uses the outer
  StateMessage branch, not the props) — reverts DataTable to its simple empty block.
- StateMessage announces errors with role=alert/aria-live=assertive (was polite for all tones).
- Remove the now-unused common.empty string (en+hu), left over from the removed QueryState render-prop.

frontend/src/components/Feed.tsx

@@ -494,7 +494,7 @@ export default function Feed({
   }
 
   if (query.isLoading) return <div className="p-8 text-muted">{t("feed.loading")}</div>;
-  if (query.isError)
+  if (query.isError && !query.data)
     return (
       <StateMessage tone="error" onRetry={() => query.refetch()}>
         {t("feed.loadError")}