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/Messages.tsx

@@ -225,7 +225,7 @@ function Directory({ onPick, onBack }: { onPick: (u: MessageUser) => void; onBac
         placeholder={t("messages.searchPeople")}
         className="w-full bg-card border border-border rounded-xl px-3 py-2 text-sm outline-none focus:border-accent"
       />
-      {q.isError ? (
+      {q.isError && !q.data ? (
         <StateMessage tone="error" onRetry={() => q.refetch()}>
           {t("common.loadError")}
         </StateMessage>