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/Messages.tsx

@@ -4,6 +4,7 @@ import { useQuery } from "@tanstack/react-query";
 import { ArrowLeft, ShieldCheck, SquarePen, ExternalLink } from "lucide-react";
 import { api, type Conversation, type MessageUser } from "../lib/api";
 import { useLiveQuery } from "../lib/useLiveQuery";
+import { StateMessage } from "./QueryState";
 import { relativeTime } from "../lib/format";
 import { partnerPub, POLL_MS, SYSTEM_ID, useKeyState } from "../lib/messaging";
 import { useHistorySubview } from "../lib/history";
@@ -130,7 +131,11 @@ function ConversationList({
         )}
       </div>
       {(keyState === "setup" || keyState === "unlock") && <KeyGate meId={meId} />}
-      {q.isLoading && !q.data ? (
+      {q.isError && !q.data ? (
+        <StateMessage tone="error" onRetry={() => q.refetch()}>
+          {t("common.loadError")}
+        </StateMessage>
+      ) : q.isLoading && !q.data ? (
         <div className="p-8 text-muted text-center">{t("common.loading")}</div>
       ) : items.length === 0 ? (
         <div className="glass rounded-2xl p-10 text-center text-muted">{t("messages.empty")}</div>
@@ -220,7 +225,11 @@ function Directory({ onPick, onBack }: { onPick: (u: MessageUser) => void; onBac
         placeholder={t("messages.searchPeople")}
         className="w-full bg-card border border-border rounded-xl px-3 py-2 text-sm outline-none focus:border-accent"
       />
-      {q.isLoading ? (
+      {q.isError ? (
+        <StateMessage tone="error" onRetry={() => q.refetch()}>
+          {t("common.loadError")}
+        </StateMessage>
+      ) : q.isLoading ? (
         <div className="p-8 text-muted text-center">{t("common.loading")}</div>
       ) : users.length === 0 ? (
         <div className="glass rounded-2xl p-10 text-center text-muted">