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/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;