feat: replace nextjs-config with environment variables (#632)

* chore: remove nextjs-config

* build fix

* update readme

* explicit nextjs env vars + remove feature-unrelated things and obsolete vars from config

* full config removal

* remove force-dynamic from pages

* compile fix

* restore claude-deleted tests

* better .env.example

---------

Co-authored-by: Igor Loskutov <igor.loskutoff@gmail.com>

www/app/lib/auth.ts

@@ -1,3 +1,5 @@
+import { assertExistsAndNonEmptyString } from "./utils";
+
 export const REFRESH_ACCESS_TOKEN_ERROR = "RefreshAccessTokenError" as const;
 // 4 min is 1 min less than default authentic value. here we assume that authentic won't be set to access tokens < 4 min
 export const REFRESH_ACCESS_TOKEN_BEFORE = 4 * 60 * 1000;