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/settings_handler.go

@@ -33,5 +33,5 @@ func (h *SettingsHandler) GetSettings(c *gin.Context) {
 		return
 	}
 
-	c.JSON(http.StatusOK, gin.H{"data": settings})
+	c.JSON(http.StatusOK, gin.H{"code": 0, "message": "success", "data": settings})
 }