refactor: simplify setup flow — eliminate redundant DB calls and login round-trips

- Add AdminExists() to UserRepository (EXISTS query, not full table scan)
- SetupInitialAdmin returns tokens directly (skip separate Login call)
- Add SetupRequest DTO to auth_dto.go (replace inline struct)
- Extract defaultEmail() helper (removes duplicated email logic)
- AuthPage uses setup tokens directly (skip redundant apiLogin call)
- Use customAxiosInstance for auth API calls (consistent with codebase)

backend/internal/adapter/output/persistence/mock/user_repository_mock.go

@@ -85,6 +85,17 @@ func (r *UserRepositoryMock) Delete(ctx context.Context, id string) error {
 	return nil
 }
 
+func (r *UserRepositoryMock) AdminExists(ctx context.Context) (bool, error) {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	for _, u := range r.users {
+		if u.Role == "admin" {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
 func (r *UserRepositoryMock) List(ctx context.Context) ([]*entity.User, error) {
 	r.mu.RLock()
 	defer r.mu.RUnlock()