Rooms are served by the fallback shell, not prerendered

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/routes/join/[code]/+page.ts

@@ -0,0 +1,2 @@
+// A room is known only at runtime: the fallback shell serves it and the client reads the code from the address.
+export const prerender = false;