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);

frontend/src/lib/api.ts

@@ -18,14 +18,18 @@ export {
   setUnauthorizedHandler,
 };
 
-// Domain modules split out of this file, re-exported so their types + the `api` façade stay
-// importable from "./api". Their method slices are spread into `api` below.
+// Domain modules split out of this file. Their TYPES are re-exported so consumers keep importing
+// them from "./api" unchanged; their method slices stay internal (`export type *`, not `export *`)
+// and reach consumers only through the `api` façade they're spread into below. Runtime value
+// exports a domain genuinely owns (e.g. plex's EMPTY_PLEX_FILTERS/plexFilterCount) are re-exported
+// explicitly.
 import { downloadsApi } from "./api/downloads";
-export * from "./api/downloads";
+export type * from "./api/downloads";
 import { plexApi } from "./api/plex";
-export * from "./api/plex";
+export type * from "./api/plex";
+export { EMPTY_PLEX_FILTERS, plexFilterCount } from "./api/plex";
 import { messagingApi } from "./api/messaging";
-export * from "./api/messaging";
+export type * from "./api/messaging";
 
 export interface Me {
   id: number;

frontend/src/lib/api/plex.ts

@@ -460,6 +460,8 @@ export const plexApi = {
       duration_seconds,
       final,
     }),
+  // Inline union (matches api.ts's VideoStatus by design) rather than importing VideoStatus — that
+  // lives in the api.ts barrel, which imports this module, so reusing it would make a type cycle.
   plexSetState: (id: string, status: "new" | "watched" | "hidden"): Promise<unknown> =>
     req(`/api/plex/item/${encodeURIComponent(id)}/state`, {
       method: "POST",