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/PlaylistsRail.tsx

@@ -210,9 +210,20 @@ export default function PlaylistsRail({
       title: t("playlists.listGroup"),
       body: (
         <>
-          {playlists.length === 0 && !listQuery.isLoading && (
-            <div className="text-xs text-muted px-1 py-1">{t("playlists.noneYet")}</div>
-          )}
+          {playlists.length === 0 &&
+            (listQuery.isError && !listQuery.data ? (
+              <button
+                type="button"
+                onClick={() => listQuery.refetch()}
+                className="text-xs text-red-400 px-1 py-1 text-left hover:underline"
+              >
+                {t("common.loadError")} — {t("common.retry")}
+              </button>
+            ) : listQuery.isLoading ? (
+              <div className="text-xs text-muted px-1 py-1">{t("common.loading")}</div>
+            ) : (
+              <div className="text-xs text-muted px-1 py-1">{t("playlists.noneYet")}</div>
+            ))}
           {playlists.length > 0 && visiblePlaylists.length === 0 && (
             <div className="text-xs text-muted px-1 py-1">{t("playlists.noMatches")}</div>
           )}