refactor(api): code-review fixes (R7 S3b review round 1)

- barrel re-exports domains with `export type *` (not `export *`) so the
  internal downloadsApi/plexApi/messagingApi slices stay behind the façade;
  plex's runtime value exports (EMPTY_PLEX_FILTERS/plexFilterCount) re-exported
  explicitly
- App.tsx: drop the now-redundant `prefs as Record<string, unknown>` self-cast
  (Me.preferences is already that type) and its `prefsRec` alias
- plex.ts: comment why plexSetState keeps its status union inline (reusing
  VideoStatus would make an api.ts↔plex.ts type cycle)

Behaviour-identical. Gate green: typecheck, eslint 0, prettier, 74 vitest.

frontend/src/App.tsx

@@ -228,9 +228,8 @@ export default function App() {
   useEffect(() => {
     const prefs = meQuery.data?.preferences;
     if (!prefs) return;
-    const prefsRec = prefs as Record<string, unknown>;
     (["feed", "plex", "playlists"] as PanelId[]).forEach((p) => {
-      const raw = prefsRec[PREF_KEY[p]];
+      const raw = prefs[PREF_KEY[p]];
       if (raw) {
         const l = normalizeLayout(p, raw);
         setPanelLayouts((prev) => ({ ...prev, [p]: l }));
@@ -245,9 +244,9 @@ export default function App() {
       setFilterCollapsedState(prefs.filterCollapsed);
       writeAccount(LS.filterCollapsed, prefs.filterCollapsed);
     }
-    if (typeof prefsRec.playlistsCollapsed === "boolean") {
-      setPlaylistsCollapsedState(prefsRec.playlistsCollapsed);
-      writeAccount(LS.playlistsCollapsed, prefsRec.playlistsCollapsed);
+    if (typeof prefs.playlistsCollapsed === "boolean") {
+      setPlaylistsCollapsedState(prefs.playlistsCollapsed);
+      writeAccount(LS.playlistsCollapsed, prefs.playlistsCollapsed);
     }
     const lang = prefs.language;
     if (typeof lang === "string" && isSupported(lang)) setLanguage(lang);