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/features.ts

@@ -0,0 +1,55 @@
+export const FEATURES = [
+  "requireLogin",
+  "privacy",
+  "browse",
+  "sendToZulip",
+  "rooms",
+] as const;
+
+export type FeatureName = (typeof FEATURES)[number];
+
+export type Features = Readonly<Record<FeatureName, boolean>>;
+
+export const DEFAULT_FEATURES: Features = {
+  requireLogin: false,
+  privacy: true,
+  browse: false,
+  sendToZulip: false,
+  rooms: false,
+} as const;
+
+function parseBooleanEnv(
+  value: string | undefined,
+  defaultValue: boolean = false,
+): boolean {
+  if (!value) return defaultValue;
+  return value.toLowerCase() === "true";
+}
+
+// WARNING: keep process.env.* as-is, next.js won't see them if you generate dynamically
+const features: Features = {
+  requireLogin: parseBooleanEnv(
+    process.env.NEXT_PUBLIC_FEATURE_REQUIRE_LOGIN,
+    DEFAULT_FEATURES.requireLogin,
+  ),
+  privacy: parseBooleanEnv(
+    process.env.NEXT_PUBLIC_FEATURE_PRIVACY,
+    DEFAULT_FEATURES.privacy,
+  ),
+  browse: parseBooleanEnv(
+    process.env.NEXT_PUBLIC_FEATURE_BROWSE,
+    DEFAULT_FEATURES.browse,
+  ),
+  sendToZulip: parseBooleanEnv(
+    process.env.NEXT_PUBLIC_FEATURE_SEND_TO_ZULIP,
+    DEFAULT_FEATURES.sendToZulip,
+  ),
+  rooms: parseBooleanEnv(
+    process.env.NEXT_PUBLIC_FEATURE_ROOMS,
+    DEFAULT_FEATURES.rooms,
+  ),
+};
+
+export const featureEnabled = (featureName: FeatureName): boolean => {
+  return features[featureName];
+};