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/stats_handler.go

@@ -24,7 +24,7 @@ func (h *StatsHandler) GetDashboard(c *gin.Context) {
 		c.JSON(http.StatusInternalServerError, gin.H{"code": 500, "message": "获取仪表盘数据失败"})
 		return
 	}
-	c.JSON(http.StatusOK, gin.H{"code": 0, "data": stats})
+	c.JSON(http.StatusOK, gin.H{"code": 0, "message": "success", "data": stats})
 }
 
 func (h *StatsHandler) GetUserStats(c *gin.Context) {
@@ -33,5 +33,5 @@ func (h *StatsHandler) GetUserStats(c *gin.Context) {
 		c.JSON(http.StatusInternalServerError, gin.H{"code": 500, "message": "获取用户统计失败"})
 		return
 	}
-	c.JSON(http.StatusOK, gin.H{"code": 0, "data": stats})
+	c.JSON(http.StatusOK, gin.H{"code": 0, "message": "success", "data": stats})
 }