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/ChatThread.tsx

@@ -5,6 +5,7 @@ import { Send } from "lucide-react";
 import { api, HttpError, type Message, type MessageUser } from "../lib/api";
 import { useLiveQuery } from "../lib/useLiveQuery";
 import { useScrollFade } from "../lib/useScrollFade";
+import { StateMessage } from "./QueryState";
 import { relativeTime } from "../lib/format";
 import { notify } from "../lib/notifications";
 import { partnerPub, POLL_MS, useKeyState } from "../lib/messaging";
@@ -139,7 +140,15 @@ export default function ChatThread({
         style={fade.style}
         className="flex-1 overflow-y-auto no-scrollbar p-3 flex flex-col gap-2"
       >
-        {items.length === 0 ? (
+        {q.isError && !q.data ? (
+          <div className="m-auto">
+            <StateMessage tone="error" onRetry={() => q.refetch()}>
+              {t("common.loadError")}
+            </StateMessage>
+          </div>
+        ) : q.isPending ? (
+          <div className="m-auto text-sm text-muted">{t("common.loading")}</div>
+        ) : items.length === 0 ? (
           <div className="m-auto text-sm text-muted">{t("messages.threadEmpty")}</div>
         ) : (
           items.map((m) => {