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/ChannelDiscovery.tsx

@@ -224,7 +224,7 @@ export default function ChannelDiscovery({
         </div>
       </PageToolbar>
       <div className="px-4 pb-4 max-w-[96rem] mx-auto">
-        {query.isError ? (
+        {query.isError && !query.data ? (
           <StateMessage tone="error" onRetry={() => query.refetch()}>
             {t("common.loadError")}
           </StateMessage>