fix(auth): use correct sign-in method for genericOAuth

Add genericOAuthClient plugin to auth client and change sign-in
call from signIn.social() to signIn.oauth2() with correct
providerId parameter.

src/app/auth/signin/page.tsx

@@ -18,8 +18,8 @@ export default function SignInPage() {
   }, [session, router])
 
   const handleSignIn = async () => {
-    await signIn.social({
-      provider: "authentik",
+    await signIn.oauth2({
+      providerId: "authentik",
       callbackURL: "/",
     })
   }

src/lib/auth-client.ts

@@ -1,5 +1,8 @@
 import { createAuthClient } from "better-auth/react";
+import { genericOAuthClient } from "better-auth/client/plugins";
 
-export const authClient = createAuthClient();
+export const authClient = createAuthClient({
+  plugins: [genericOAuthClient()],
+});
 
 export const { useSession, signIn, signOut } = authClient;