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/i18n/locales/en/common.json

@@ -19,6 +19,5 @@
   "demoSharedNotice": "You're in the shared demo account. Everything you do here — playlists, hidden videos, settings — is communal and may be changed by other demo visitors at the same time.",
   "backToTop": "Back to top",
   "retry": "Retry",
-  "loadError": "Couldn't load this — check your connection and try again.",
-  "empty": "Nothing here yet."
+  "loadError": "Couldn't load this — check your connection and try again."
 }