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/vk.md

@@ -0,0 +1,57 @@
+---
+id: vk
+title: VK
+---
+
+## Documentation
+
+https://vk.com/dev/first_guide
+
+## Configuration
+
+https://vk.com/apps?act=manage
+
+## Options
+
+The **VK Provider** comes with a set of default options:
+
+- [VK Provider options](https://github.com/nextauthjs/next-auth/blob/v4/packages/next-auth/src/providers/vk.ts)
+
+You can override any of the options to suit your own use case.
+
+## Example
+
+```js
+import VkProvider from "next-auth/providers/vk";
+...
+providers: [
+  VkProvider({
+    clientId: process.env.VK_CLIENT_ID,
+    clientSecret: process.env.VK_CLIENT_SECRET
+  })
+]
+...
+```
+
+:::note
+By default the provider uses `5.131` version of the API. See https://vk.com/dev/versions for more info.
+:::
+
+If you want to use a different version, you can pass it to provider's options object:
+
+```js
+// pages/api/auth/[...nextauth].js
+
+const apiVersion = "5.131"
+...
+providers: [
+  VkProvider({
+    accessTokenUrl: `https://oauth.vk.com/access_token?v=${apiVersion}`,
+    requestTokenUrl: `https://oauth.vk.com/access_token?v=${apiVersion}`,
+    authorizationUrl:
+      `https://oauth.vk.com/authorize?response_type=code&v=${apiVersion}`,
+    profileUrl: `https://api.vk.com/method/users.get?fields=photo_100&v=${apiVersion}`,
+  })
+]
+...
+```