refactor: add SubNav layout and per-subsite nav placeholders; switch Base to use SubNav

src/pages/radio.astro

@@ -1,6 +1,7 @@
 ---
 import Base from "../layouts/Base.astro";
 import Root from "../components/AppLayout.jsx";
+// The Base layout exposes runtime subsite state — no per-page detection needed
 import { requireAuthHook } from "@/hooks/requireAuthHook";
 const user = await requireAuthHook(Astro);
 ---