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/input/http/dto/auth_dto.go

@@ -50,6 +50,13 @@ type LoginRequest struct {
 	Password string `json:"password" binding:"required"`
 }
 
+// SetupRequest 初始管理员注册请求
+type SetupRequest struct {
+	Username string `json:"username" binding:"required"`
+	Password string `json:"password" binding:"required"`
+	Email    string `json:"email,omitempty"`
+}
+
 // RefreshTokenRequest 刷新 Token 请求
 type RefreshTokenRequest struct {
 	RefreshToken string `json:"refreshToken" binding:"required"`