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/AdminUsers.tsx

@@ -7,6 +7,7 @@ import { notify } from "../lib/notifications";
 import { ADMIN_USERS_TAB_KEY } from "../lib/adminUsersTab";
 import Tooltip from "./Tooltip";
 import { Section } from "./ui/form";
+import { StateMessage } from "./QueryState";
 import { useConfirm } from "./ConfirmProvider";
 import Tabs, { usePersistedTab } from "./Tabs";
 import { PageToolbar } from "./PageShell";
@@ -159,7 +160,13 @@ function UsersRoles({ me }: { me: Me }) {
   return (
     <Section card title={t("users.roles.title")}>
       <p className="text-xs text-muted leading-relaxed mb-3">{t("users.roles.intro")}</p>
-      {rows.length === 0 ? (
+      {q.isError && !q.data ? (
+        <StateMessage tone="error" onRetry={() => q.refetch()}>
+          {t("common.loadError")}
+        </StateMessage>
+      ) : q.isLoading && !q.data ? (
+        <p className="text-sm text-muted">{t("common.loading")}</p>
+      ) : rows.length === 0 ? (
         <p className="text-sm text-muted">{t("users.roles.empty")}</p>
       ) : (
         <div className="space-y-1.5">
@@ -307,7 +314,13 @@ function AdminInvites() {
 
   return (
     <Section card title={t("settings.invites.title")}>
-      {pending.length === 0 ? (
+      {invites.isError && !invites.data ? (
+        <StateMessage tone="error" onRetry={() => invites.refetch()}>
+          {t("common.loadError")}
+        </StateMessage>
+      ) : invites.isLoading && !invites.data ? (
+        <p className="text-sm text-muted">{t("common.loading")}</p>
+      ) : pending.length === 0 ? (
         <p className="text-sm text-muted">{t("settings.invites.noPending")}</p>
       ) : (
         <div className="space-y-1.5">