feat: refine navigation and account access flow

src/plugins/auth.js

@@ -93,6 +93,13 @@ async function authPlugin(fastify, { config, googleClient } = {}) {
         path: '/',
       },
     });
+
+    fastify.get('/auth/google/select-account', async (request, reply) => {
+      const authorizationUri = await fastify.googleOAuth2.generateAuthorizationUri(request, reply);
+      const url = new URL(authorizationUri);
+      url.searchParams.set('prompt', 'select_account');
+      return reply.redirect(url.href);
+    });
   } else {
     fastify.get('/auth/google', async () => {
       throw new AppError('Google OAuth가 설정되지 않았습니다.', {
@@ -133,10 +140,7 @@ async function authPlugin(fastify, { config, googleClient } = {}) {
     }
     const allowedAccount = await fastify.allowedEmailRepository.findByEmail(payload.email);
     if (!allowedAccount) {
-      throw new AppError('허용되지 않은 Google 계정입니다.', {
-        statusCode: 403,
-        code: 'GOOGLE_EMAIL_NOT_ALLOWED',
-      });
+      return reply.redirect('/login?error=account-not-allowed');
     }
 
     const user = await fastify.userRepository.upsertGoogleUser({