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/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;