feat(ui): R3 S1 — honest error states across the pages

The error state didn't exist as a concept: a failed main query rendered as an "empty" list
("No channels" while the request actually died), so a backend restart read as "everything is
empty". Add the convention that an empty state may NEVER show while a query is erroring.

- New components/QueryState.tsx: shared StateMessage (loading / error+Retry, house style, with
  role=status/aria-live — the app had zero live regions).
- DataTable gains loading/error/onRetry props (suppresses empty-text on error).
- isError branch (before the empty branch) added across the false-empty pages: Feed (retry),
  Channels, ChannelDiscovery, AuditLog, NotificationsPanel, Messages (conversations + people),
  Scheduler, Stats, PlexBrowse grid, PlexPlaylistView. ChannelPage's main content is Feed, now
  covered. common.loadError/empty strings (en+hu).

frontend/src/components/AuditLog.tsx

@@ -8,6 +8,7 @@ import { LS } from "../lib/storage";
 import { Section } from "./ui/form";
 import { useConfirm } from "./ConfirmProvider";
 import DataTable from "./DataTable";
+import { StateMessage } from "./QueryState";
 import { auditColumns } from "./auditColumns";
 
 // Admin-only audit trail: an append-only who/what/when log of admin actions + scheduler run
@@ -53,7 +54,11 @@ export default function AuditLog() {
     <div className="p-4 max-w-5xl w-full mx-auto">
       <Section card title={t("audit.title")}>
         <p className="text-xs text-muted leading-relaxed mb-3">{t("audit.intro")}</p>
-        {q.isLoading ? (
+        {q.isError ? (
+          <StateMessage tone="error" onRetry={() => q.refetch()}>
+            {t("common.loadError")}
+          </StateMessage>
+        ) : q.isLoading ? (
           <div className="text-muted py-8">{t("common.loading")}</div>
         ) : rows.length === 0 ? (
           <p className="text-sm text-muted">{t("audit.empty")}</p>