feat(ui): R3 S2 — loading rows for blank-flash pages, error paths for forever-loading ones

- Blank-flash (showed an empty state on first load before data): DownloadCenter (queue + library
  tabs), AdminUsers (roles + invites), ChatThread, PlaylistsRail now show a loading indicator
  (and an error+retry) instead of a false-empty flash — guarded on `&& !data` so live data isn't
  disturbed on refetch.
- Forever-loading (`isLoading || !data` loops forever on a failed fetch): ConfigPanel, the three
  PlexBrowse subviews (playlist/show/season), and Stats' admin dashboard get an `isError && !data`
  error+retry path before the loading branch.
- Playlists' detail pane already had an error path (isError+retry); its list lives in PlaylistsRail
  (now covered), so no change needed there.

frontend/src/components/PlexBrowse.tsx

@@ -599,7 +599,11 @@ function PlexInfoView({
           {t("plex.backToLibrary")}
         </button>
       </div>
-      {q.isLoading || !q.data ? (
+      {q.isError && !q.data ? (
+        <StateMessage tone="error" onRetry={() => q.refetch()}>
+          {t("common.loadError")}
+        </StateMessage>
+      ) : q.isLoading || !q.data ? (
         <p className="p-8 text-muted text-sm">{t("plex.loading")}</p>
       ) : (
         <Suspense fallback={<p className="p-8 text-muted text-sm">{t("plex.loading")}</p>}>
@@ -1018,7 +1022,11 @@ function PlexShowView({
     <div className="p-4 max-w-[1200px] mx-auto glass-media">
       <BackBtn onBack={onBack} label={t("plex.backToLibrary")} />
 
-      {q.isLoading || !d || !show ? (
+      {q.isError && !q.data ? (
+        <StateMessage tone="error" onRetry={() => q.refetch()}>
+          {t("common.loadError")}
+        </StateMessage>
+      ) : q.isLoading || !d || !show ? (
         <p className="text-muted text-sm">{t("plex.loading")}</p>
       ) : (
         <>
@@ -1250,7 +1258,11 @@ function PlexSeasonView({
     <div className="p-4 max-w-[1200px] mx-auto glass-media">
       <BackBtn onBack={onBack} label={t("plex.series.backToShow")} />
 
-      {q.isLoading || !d || !se ? (
+      {q.isError && !q.data ? (
+        <StateMessage tone="error" onRetry={() => q.refetch()}>
+          {t("common.loadError")}
+        </StateMessage>
+      ) : q.isLoading || !d || !se ? (
         <p className="text-muted text-sm">{t("plex.loading")}</p>
       ) : (
         <>