fix: unify handler response format in multiple handlers

- captcha_handler.go: Fix GenerateCaptcha/VerifyCaptcha to use {code, message, data}
- password_reset_handler.go: Fix all error responses to use {code, message}
- settings_handler.go: Add missing "code" and "message" fields
- sms_handler.go: Fix error responses to use {code, message}
- sso_handler.go: Fix all error responses to use {code, message, data}
- stats_handler.go: Add missing "message" field in success responses
- theme_handler.go: Fix error responses to use {code, message}
- totp_handler.go: Fix all responses to use {code, message, data}

Standardize all JSON responses to {code: 0, message: "success", data: ...} for success
and {code: XXX, message: "..."} for errors.

internal/api/handler/sms_handler.go

@@ -30,13 +30,13 @@ func NewSMSHandlerWithService(authService *service.AuthService, smsCodeService *
 // SendCode 发送短信验证码（用于注册/登录）
 func (h *SMSHandler) SendCode(c *gin.Context) {
 	if h.smsCodeService == nil {
-		c.JSON(http.StatusServiceUnavailable, gin.H{"error": "SMS service not configured"})
+		c.JSON(http.StatusServiceUnavailable, gin.H{"code": 503, "message": "SMS service not configured"})
 		return
 	}
 
 	var req service.SendCodeRequest
 	if err := c.ShouldBindJSON(&req); err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		c.JSON(http.StatusBadRequest, gin.H{"code": 400, "message": err.Error()})
 		return
 	}
 
@@ -56,7 +56,7 @@ func (h *SMSHandler) SendCode(c *gin.Context) {
 // LoginByCode 短信验证码登录（带设备信息以支持设备信任链路）
 func (h *SMSHandler) LoginByCode(c *gin.Context) {
 	if h.authService == nil {
-		c.JSON(http.StatusServiceUnavailable, gin.H{"error": "SMS login not configured"})
+		c.JSON(http.StatusServiceUnavailable, gin.H{"code": 503, "message": "SMS login not configured"})
 		return
 	}
 
@@ -70,7 +70,7 @@ func (h *SMSHandler) LoginByCode(c *gin.Context) {
 	}
 
 	if err := c.ShouldBindJSON(&req); err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		c.JSON(http.StatusBadRequest, gin.H{"code": 400, "message": err.Error()})
 		return
 	}