fix: 修复验证码请求429错误

问题：
- 短时间内多次请求验证码触发限流（429 Too Many Requests）

修复：
- 后端：验证码最小请求间隔从3秒改为1秒
- 前端：添加2秒防抖，避免重复请求
- 前端：429错误时保留已有验证码图片

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/app.js

@@ -622,52 +622,48 @@ handleDragLeave(e) {
       }
     },
 
-    // 刷新验证码（登录）
-    async refreshCaptcha() {
+    // 通用验证码加载函数（带防抖）
+    async loadCaptcha(targetField) {
+      // 防抖：2秒内不重复请求
+      const now = Date.now();
+      if (this._lastCaptchaTime && (now - this._lastCaptchaTime) < 2000) {
+        console.log('[验证码] 请求过于频繁，跳过');
+        return;
+      }
+      this._lastCaptchaTime = now;
+
       try {
-        const response = await axios.get(`${this.apiBase}/api/captcha?t=${Date.now()}`, {
+        const response = await axios.get(`${this.apiBase}/api/captcha?t=${now}`, {
           responseType: 'blob'
         });
-        this.captchaUrl = URL.createObjectURL(response.data);
+        this[targetField] = URL.createObjectURL(response.data);
       } catch (error) {
         console.error('获取验证码失败:', error);
+        // 如果是429错误，不清除已有验证码
+        if (error.response?.status !== 429) {
+          this[targetField] = '';
+        }
       }
     },
 
+    // 刷新验证码（登录）
+    refreshCaptcha() {
+      this.loadCaptcha('captchaUrl');
+    },
+
     // 刷新注册验证码
-    async refreshRegisterCaptcha() {
-      try {
-        const response = await axios.get(`${this.apiBase}/api/captcha?t=${Date.now()}`, {
-          responseType: 'blob'
-        });
-        this.registerCaptchaUrl = URL.createObjectURL(response.data);
-      } catch (error) {
-        console.error('获取验证码失败:', error);
-      }
+    refreshRegisterCaptcha() {
+      this.loadCaptcha('registerCaptchaUrl');
     },
 
     // 刷新忘记密码验证码
-    async refreshForgotPasswordCaptcha() {
-      try {
-        const response = await axios.get(`${this.apiBase}/api/captcha?t=${Date.now()}`, {
-          responseType: 'blob'
-        });
-        this.forgotPasswordCaptchaUrl = URL.createObjectURL(response.data);
-      } catch (error) {
-        console.error('获取验证码失败:', error);
-      }
+    refreshForgotPasswordCaptcha() {
+      this.loadCaptcha('forgotPasswordCaptchaUrl');
     },
 
     // 刷新重发验证邮件验证码
-    async refreshResendVerifyCaptcha() {
-      try {
-        const response = await axios.get(`${this.apiBase}/api/captcha?t=${Date.now()}`, {
-          responseType: 'blob'
-        });
-        this.resendVerifyCaptchaUrl = URL.createObjectURL(response.data);
-      } catch (error) {
-        console.error('获取验证码失败:', error);
-      }
+    refreshResendVerifyCaptcha() {
+      this.loadCaptcha('resendVerifyCaptchaUrl');
     },
 
     async resendVerification() {