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/hubspot.md

@@ -0,0 +1,42 @@
+---
+id: hubspot
+title: HubSpot
+---
+
+:::note
+HubSpot returns a limited amount of information on the token holder (see [docs](https://legacydocs.hubspot.com/docs/methods/oauth2/get-access-token-information)). One other issue is that the name and profile photo cannot be fetched through API as discussed [here](https://community.hubspot.com/t5/APIs-Integrations/Profile-photo-is-not-retrieved-with-User-API/m-p/325521).
+:::
+
+## Documentation
+
+https://developers.hubspot.com/docs/api/oauth-quickstart-guide
+
+## Configuration
+
+You need to have an APP in your Developer Account as described at https://developers.hubspot.com/docs/api/developer-tools-overview
+
+## Options
+
+The **HubSpot Provider** comes with a set of default options:
+
+- [HubSpot Provider options](https://github.com/nextauthjs/next-auth/blob/v4/packages/next-auth/src/providers/hubspot.ts)
+
+You can override any of the options to suit your own use case.
+
+## Example
+
+```js
+import HubspotProvider from "next-auth/providers/hubspot";
+...
+providers: [
+  HubspotProvider({
+    clientId: process.env.HUBSPOT_CLIENT_ID,
+    clientSecret: process.env.HUBSPOT_CLIENT_SECRET
+  })
+]
+...
+```
+
+:::warning
+The **Redirect URL** under the **Auth** tab on the HubSpot App Settings page must match the callback url which would be http://localhost:3000/api/auth/callback/hubspot for local development. Only one callback URL per Client ID and Client Secret pair is allowed, so it might be easier to create a new app for local development then fiddle with the url changes.  
+:::