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/app/admin/settings/cms-settings-config.ts

@@ -1,4 +1,3 @@
-import { FALLBACK_HOTEL_NAME } from "@/lib/brand";
 import { HABBO_GAMEDATA_HOTELS } from "@/lib/habbo-gamedata-hotel";
 
 export type FieldType =
@@ -50,7 +49,7 @@ export const SETTINGS_GROUPS: SettingsGroup[] = [
 				label: "Hotel name",
 				type: "text",
 				placeholder: "Epicnabbo",
-				defaultValue: FALLBACK_HOTEL_NAME,
+				defaultValue: "",
 			},
 			{
 				key: "cms_logo",