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/captcha_handler.go

@@ -26,13 +26,17 @@ func (h *CaptchaHandler) GenerateCaptcha(c *gin.Context) {
 	}
 
 	c.JSON(http.StatusOK, gin.H{
-		"captcha_id": result.CaptchaID,
-		"image":      result.ImageData,
+		"code":    0,
+		"message": "success",
+		"data": gin.H{
+			"captcha_id": result.CaptchaID,
+			"image":      result.ImageData,
+		},
 	})
 }
 
 func (h *CaptchaHandler) GetCaptchaImage(c *gin.Context) {
-	c.JSON(http.StatusOK, gin.H{"message": "captcha image endpoint"})
+	c.JSON(http.StatusOK, gin.H{"code": 0, "message": "success"})
 }
 
 func (h *CaptchaHandler) VerifyCaptcha(c *gin.Context) {
@@ -42,13 +46,13 @@ func (h *CaptchaHandler) VerifyCaptcha(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
 	}
 
 	if h.captchaService.Verify(c.Request.Context(), req.CaptchaID, req.Answer) {
-		c.JSON(http.StatusOK, gin.H{"verified": true})
+		c.JSON(http.StatusOK, gin.H{"code": 0, "message": "success", "data": gin.H{"verified": true}})
 	} else {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "invalid captcha"})
+		c.JSON(http.StatusBadRequest, gin.H{"code": 400, "message": "invalid captcha"})
 	}
 }