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/QueryState.tsx

@@ -0,0 +1,48 @@
+import type { ReactNode } from "react";
+import { useTranslation } from "react-i18next";
+import { AlertTriangle } from "lucide-react";
+
+// Centered inline status line for a page/section's loading and error(+retry) states, in the house
+// style (mirrors Playlists' error block). Text-only by design — R3's goal is to stop pages
+// rendering an error or a still-loading fetch AS an "empty" state, not to add skeletons. The
+// `role=status`/`aria-live` also gives screen readers the state change (the app had zero live
+// regions before).
+export function StateMessage({
+  children,
+  onRetry,
+  tone = "muted",
+  className = "",
+}: {
+  children: ReactNode;
+  onRetry?: () => void;
+  tone?: "muted" | "error";
+  className?: string;
+}) {
+  const { t } = useTranslation();
+  return (
+    <div
+      className={`flex flex-col items-center gap-3 py-10 text-sm text-muted ${className}`}
+      role="status"
+      aria-live="polite"
+    >
+      <span className="inline-flex items-center gap-2 text-center">
+        {tone === "error" && <AlertTriangle className="w-4 h-4 shrink-0 text-red-400" />}
+        {children}
+      </span>
+      {onRetry && (
+        <button
+          type="button"
+          onClick={onRetry}
+          className="px-3 py-1.5 rounded-lg border border-border text-xs text-muted hover:text-fg hover:border-accent transition"
+        >
+          {t("common.retry")}
+        </button>
+      )}
+    </div>
+  );
+}
+
+// The honest-state convention every page now follows: check `isError` BEFORE the empty branch so a
+// failed fetch renders this message (with Retry) instead of the page's "empty" text — the R3 bug
+// where a backend restart read as "No channels". `StateMessage` is the shared building block;
+// pages wire it into their existing loading/empty ternary (and DataTable takes loading/error props).