fix: 修复验证码session不一致的问题

问题原因：
- 验证码图片通过<img src>加载，可能不携带session cookie
- 导致验证码生成和表单提交使用不同的session

修复方案：
- 改用axios请求获取验证码（blob格式）
- 确保验证码请求携带withCredentials
- 点击"忘记密码"时立即加载验证码
- 切换到注册模式时立即加载验证码

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

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

frontend/app.html

@@ -1048,7 +1048,7 @@
             </div>
           </div>
           <div style="text-align: right; margin-bottom: 15px;">
-            <a @click="showForgotPasswordModal = true" style="color: #667eea; cursor: pointer; font-size: 14px; text-decoration: none;">
+            <a @click="showForgotPasswordModal = true; refreshForgotPasswordCaptcha()" style="color: #667eea; cursor: pointer; font-size: 14px; text-decoration: none;">
               忘记密码？
             </a>
           </div>

frontend/app.js

@@ -516,6 +516,10 @@ handleDragLeave(e) {
       this.isLogin = !this.isLogin;
       this.errorMessage = '';
       this.successMessage = '';
+      // 切换到注册模式时加载验证码
+      if (!this.isLogin) {
+        this.refreshRegisterCaptcha();
+      }
     },
 
     async handleLogin() {
@@ -619,23 +623,51 @@ handleDragLeave(e) {
     },
 
     // 刷新验证码（登录）
-    refreshCaptcha() {
-      this.captchaUrl = `${this.apiBase}/api/captcha?t=${Date.now()}`;
+    async refreshCaptcha() {
+      try {
+        const response = await axios.get(`${this.apiBase}/api/captcha?t=${Date.now()}`, {
+          responseType: 'blob'
+        });
+        this.captchaUrl = URL.createObjectURL(response.data);
+      } catch (error) {
+        console.error('获取验证码失败:', error);
+      }
     },
 
     // 刷新注册验证码
-    refreshRegisterCaptcha() {
-      this.registerCaptchaUrl = `${this.apiBase}/api/captcha?t=${Date.now()}`;
+    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);
+      }
     },
 
     // 刷新忘记密码验证码
-    refreshForgotPasswordCaptcha() {
-      this.forgotPasswordCaptchaUrl = `${this.apiBase}/api/captcha?t=${Date.now()}`;
+    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);
+      }
     },
 
     // 刷新重发验证邮件验证码
-    refreshResendVerifyCaptcha() {
-      this.resendVerifyCaptchaUrl = `${this.apiBase}/api/captcha?t=${Date.now()}`;
+    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);
+      }
     },
 
     async resendVerification() {