feat(auth): add Google OAuth login

Support Google login that links to existing accounts by email.
When a user who registered via email OTP signs in with Google using
the same email, they are linked to the same account.

Backend:
- Add POST /auth/google endpoint that exchanges Google auth code for
  tokens, fetches user profile, and calls findOrCreateUser()
- Updates user name and avatar from Google profile on first Google login

Frontend:
- Add "Continue with Google" button on login page (shown when
  NEXT_PUBLIC_GOOGLE_CLIENT_ID is configured)
- Add /auth/callback page to handle Google OAuth redirect
- Add loginWithGoogle to auth store and API client

apps/web/shared/api/client.ts

@@ -144,6 +144,13 @@ export class ApiClient {
     });
   }
 
+  async googleLogin(code: string, redirectUri: string): Promise<LoginResponse> {
+    return this.fetch("/auth/google", {
+      method: "POST",
+      body: JSON.stringify({ code, redirect_uri: redirectUri }),
+    });
+  }
+
   async getMe(): Promise<User> {
     return this.fetch("/api/me");
   }