chore(auth): update NextAuth configuration to include JWT language

chore(eslint): add warning rule for unused variables in ESLint configuration

.eslintrc.json

@@ -1,6 +1,7 @@
 {
   "extends": ["next/core-web-vitals", "next/typescript"],
   "rules": {
+    "@typescript-eslint/no-unused-vars": "warn",
     "@typescript-eslint/no-empty-object-type": "warn"
   }
 }

src/auth.ts

@@ -1,13 +1,14 @@
-import NextAuth, { type DefaultSession } from "next-auth";
+import NextAuth from "next-auth";
+import { JWT } from "next-auth/jwt";
 import Gitea, { GiteaProfile } from "@/lib/providers/gitea";
 
 declare module "next-auth" {
   interface User extends GiteaProfile {}
+}
 
-  interface Session {
-    user: {
-      language: string;
-    } & DefaultSession["user"];
+declare module "next-auth/jwt" {
+  interface JWT {
+    language?: string;
   }
 }
 
@@ -30,8 +31,9 @@ export const { handlers, signIn, signOut, auth } = NextAuth({
       return token;
     },
     session({ session, token }) {
-      session.user.language = token.language as string;
+      session.user.language = token.language;
       return session;
     },
   },
+  debug: process.env.NODE_ENV === "development",
 });