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/i18n/locales/en/common.json

@@ -18,5 +18,7 @@
   "demoTitle": "Demo account",
   "demoSharedNotice": "You're in the shared demo account. Everything you do here — playlists, hidden videos, settings — is communal and may be changed by other demo visitors at the same time.",
   "backToTop": "Back to top",
-  "retry": "Retry"
+  "retry": "Retry",
+  "loadError": "Couldn't load this — check your connection and try again.",
+  "empty": "Nothing here yet."
 }

frontend/src/i18n/locales/hu/common.json

@@ -18,5 +18,7 @@
   "demoTitle": "Demo fiók",
   "demoSharedNotice": "Egy közös demo fiókban vagy. Minden, amit itt csinálsz — lejátszási listák, elrejtett videók, beállítások — közös, és más demo látogatók egyszerre módosíthatják.",
   "backToTop": "Vissza a tetejére",
-  "retry": "Újra"
+  "retry": "Újra",
+  "loadError": "Nem sikerült betölteni — ellenőrizd a kapcsolatot, és próbáld újra.",
+  "empty": "Itt még nincs semmi."
 }