feat(auth): extend NextAuth user and session to include language support

refactor(gitea): make username optional in GiteaProfile interface

style(eslint): add warning rule for empty object type in ESLint configuration

.eslintrc.json

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

src/auth.ts

-import NextAuth from "next-auth";
-import Gitea from "@/lib/providers/gitea";
+import NextAuth, { type DefaultSession } from "next-auth";
+import Gitea, { GiteaProfile } from "@/lib/providers/gitea";
+
+declare module "next-auth" {
+  interface User extends GiteaProfile {}
+
+  interface Session {
+    user: {
+      language: string;
+    } & DefaultSession["user"];
+  }
+}
 
 export const { handlers, signIn, signOut, auth } = NextAuth({
   providers: [
@@ -12,4 +22,16 @@ export const { handlers, signIn, signOut, auth } = NextAuth({
     }),
   ],
   secret: process.env.AUTH_SECRET,
+  callbacks: {
+    jwt({ token, user }) {
+      if (user) {
+        token.language = user.language;
+      }
+      return token;
+    },
+    session({ session, token }) {
+      session.user.language = token.language as string;
+      return session;
+    },
+  },
 });

src/lib/providers/gitea.ts

@@ -2,7 +2,7 @@ import { Email as GiteaEmail, User } from "gitea-js";
 import { OAuthConfig, OAuthUserConfig } from "next-auth/providers";
 
 export interface GiteaProfile extends User {
-  username: string;
+  username?: string;
 }
 
 export default function Gitea(
@@ -65,12 +65,13 @@ export default function Gitea(
         name: profile.username ?? profile.login,
         email: profile.email,
         image: profile.avatar_url,
+        language: profile.language,
       };
     },
     options: config,
-    style:{
-      logo:"/gitea.svg",
-      brandColor:"#609926",
-    }
+    style: {
+      logo: "/gitea.svg",
+      brandColor: "#609926",
+    },
   };
 }