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/battlenet.md

@@ -0,0 +1,46 @@
+---
+id: battle.net
+title: Battle.net
+---
+
+## Documentation
+
+https://develop.battle.net/documentation/guides/using-oauth
+
+## Configuration
+
+https://develop.battle.net/access/clients
+
+## Options
+
+The **Battle.net Provider** comes with a set of default options:
+
+- [Battle.net Provider options](https://github.com/nextauthjs/next-auth/blob/v4/packages/next-auth/src/providers/battlenet.js)
+
+You can override any of the options to suit your own use case.
+
+## Example
+
+```js
+import BattleNetProvider from "next-auth/providers/battlenet";
+...
+providers: [
+  BattleNetProvider({
+    clientId: process.env.BATTLENET_CLIENT_ID,
+    clientSecret: process.env.BATTLENET_CLIENT_SECRET,
+    issuer: process.env.BATTLENET_ISSUER
+  })
+]
+...
+```
+
+`issuer` must be one of these values, based on the [available regions](https://develop.battle.net/documentation/guides/regionality-and-apis):
+
+```ts
+type BattleNetIssuer =
+  | "https://www.battlenet.com.cn/oauth"
+  | "https://us.battle.net/oauth"
+  | "https://eu.battle.net/oauth"
+  | "https://kr.battle.net/oauth"
+  | "https://tw.battle.net/oauth"
+```