refactor: remove hotel-name fallback, fail fast when unconfigured

Drop the hardcoded FALLBACK_HOTEL_NAME ("Atom") preset and the brand.ts
module. HOTEL_NAME is now a required env var: if it (and the CMS hotel_name
setting) is missing the site fails validation at startup/build with a clear
message instead of silently rendering a placeholder hotel name.

resolveHotelName() resolves CMS hotel_name -> required HOTEL_NAME only.
Callers that used the preset (api/home route catch branch, CMS settings form
default, mobile-nav/logo-generator prop defaults) now use the configured name
or an empty default; the real name is already passed in by server parents.

src/env.ts

@@ -1,5 +1,4 @@
 import { z } from "zod";
-import { FALLBACK_HOTEL_NAME } from "@/lib/brand";
 
 // Minimal validated env for the foundation. When the Next.js app is added this
 // will move to @t3-oss/env-nextjs (the habbo-next pattern), but the data layer
@@ -25,7 +24,12 @@ const schema = z
 			.int()
 			.positive()
 			.default(10_000),
-		HOTEL_NAME: z.string().default(FALLBACK_HOTEL_NAME),
+		HOTEL_NAME: z
+			.string()
+			.min(
+				1,
+				"HOTEL_NAME is not set — the site has not been configured/built yet.",
+			),
 		APP_URL: z.string().url().default("http://localhost:3000"),
 		NEXT_PUBLIC_APP_URL: z.string().url().default("http://localhost:3000"),
 		// Public imager URL — overrides the default /imaging relative path.