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/DownloadCenter.tsx

@@ -19,6 +19,7 @@ import {
 import clsx from "clsx";
 import Tabs, { usePersistedTab } from "./Tabs";
 import { PageToolbar } from "./PageShell";
+import { StateMessage } from "./QueryState";
 import Modal from "./Modal";
 // Lazy: these dialogs/editors open on demand, so they stay out of the Downloads page chunk until
 // used (the video editor in particular is heavy — filmstrip + scrubber).
@@ -803,11 +804,18 @@ export default function DownloadCenter() {
                   </IconBtn>
                 </DownloadRow>
               ))}
-              {queue.length === 0 && (
-                <div className="text-sm text-muted py-10 text-center">
-                  {t("downloads.empty.queue")}
-                </div>
-              )}
+              {queue.length === 0 &&
+                (jobsQ.isError && !jobsQ.data ? (
+                  <StateMessage tone="error" onRetry={() => jobsQ.refetch()}>
+                    {t("common.loadError")}
+                  </StateMessage>
+                ) : jobsQ.isLoading ? (
+                  <div className="text-sm text-muted py-10 text-center">{t("common.loading")}</div>
+                ) : (
+                  <div className="text-sm text-muted py-10 text-center">
+                    {t("downloads.empty.queue")}
+                  </div>
+                ))}
             </div>
           </>
         )}
@@ -860,11 +868,18 @@ export default function DownloadCenter() {
                   </IconBtn>
                 </DownloadRow>
               ))}
-              {library.length === 0 && (
-                <div className="text-sm text-muted py-10 text-center">
-                  {t("downloads.empty.done")}
-                </div>
-              )}
+              {library.length === 0 &&
+                (jobsQ.isError && !jobsQ.data ? (
+                  <StateMessage tone="error" onRetry={() => jobsQ.refetch()}>
+                    {t("common.loadError")}
+                  </StateMessage>
+                ) : jobsQ.isLoading ? (
+                  <div className="text-sm text-muted py-10 text-center">{t("common.loading")}</div>
+                ) : (
+                  <div className="text-sm text-muted py-10 text-center">
+                    {t("downloads.empty.done")}
+                  </div>
+                ))}
             </div>
           </>
         )}