refactor(auth): switch to class-based social login and v4 route handler

Migrate from function-based social login helpers (`facebookLogin`, `googleLogin`)
to class-based services (`new FacebookLogin().signIn()`, `new GoogleLogin().signIn()`),
updating call sites in `AuthFacebookPanel` and `SplashButton` with try/catch error
handling. The NextAuth route handler is also refactored to the v4 pattern, importing
pre-built `GET` / `POST` exports from `@/lib/auth/nextauth` instead of constructing
the handler inline with `NextAuth(authOptions)`.

docs/nextauth/docs/providers/naver.md

@@ -0,0 +1,34 @@
+---
+id: naver
+title: Naver
+---
+
+## Documentation
+
+https://developers.naver.com/docs/login/overview/overview.md
+
+## Configuration
+
+https://developers.naver.com/docs/login/api/api.md
+
+## Options
+
+The **Naver Provider** comes with a set of default options:
+
+- [Naver Provider options](https://github.com/nextauthjs/next-auth/blob/v4/packages/next-auth/src/providers/naver.ts)
+
+You can override any of the options to suit your own use case.
+
+## Example
+
+```js
+import NaverProvider from "next-auth/providers/naver";
+...
+providers: [
+  NaverProvider({
+    clientId: process.env.NAVER_CLIENT_ID,
+    clientSecret: process.env.NAVER_CLIENT_SECRET
+  })
+]
+...
+```