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/Channels.tsx

@@ -710,7 +710,7 @@ 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.isError ? (
+          {channelsQuery.isError && !channelsQuery.data ? (
             <StateMessage tone="error" onRetry={() => channelsQuery.refetch()}>
               {t("common.loadError")}
             </StateMessage>