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/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",