feat: 注册、重置密码、重发验证邮件添加验证码功能

后端修改：
- 添加通用验证码验证函数 verifyCaptcha()
- /api/register 接口添加验证码验证
- /api/password/forgot 接口添加验证码验证
- /api/resend-verification 接口添加验证码验证

前端修改：
- 注册表单添加验证码输入框和图片
- 忘记密码模态框添加验证码
- 重发验证邮件区域添加验证码输入
- 添加各表单的验证码刷新方法
- 提交失败后自动刷新验证码

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

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

frontend/app.html

@@ -1038,7 +1038,14 @@
             <small style="color: var(--text-muted); font-size: 12px;">点击图片刷新验证码</small>
           </div>
           <div v-if="showResendVerify" class="alert alert-info" style="margin-bottom: 10px;">
-            邮箱未验证？<a style="color:#667eea; cursor: pointer;" @click="resendVerification">点击重发激活邮件</a>
+            <div>邮箱未验证？请输入验证码后重发激活邮件</div>
+            <div style="display: flex; gap: 10px; align-items: center; margin-top: 8px;">
+              <input type="text" class="form-input" v-model="resendVerifyCaptcha" placeholder="验证码" style="flex: 1; height: 40px;" @focus="!resendVerifyCaptchaUrl && refreshResendVerifyCaptcha()">
+              <img v-if="resendVerifyCaptchaUrl" :src="resendVerifyCaptchaUrl" @click="refreshResendVerifyCaptcha" style="height: 40px; cursor: pointer; border-radius: 4px;" title="点击刷新验证码">
+              <button type="button" class="btn btn-primary" @click="resendVerification" style="height: 40px; white-space: nowrap;">
+                重发邮件
+              </button>
+            </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;">
@@ -1049,7 +1056,7 @@
             <i class="fas fa-right-to-bracket"></i> 登录
           </button>
         </form>
-        <form v-else @submit.prevent="handleRegister">
+        <form v-else @submit.prevent="handleRegister" @focusin="!registerCaptchaUrl && refreshRegisterCaptcha()">
           <div class="form-group">
             <label class="form-label">用户名(3-20字符)</label>
             <input type="text" class="form-input" v-model="registerForm.username" required minlength="3" maxlength="20">
@@ -1062,6 +1069,13 @@
             <label class="form-label">密码 (至少6字符)</label>
             <input type="password" class="form-input" v-model="registerForm.password" required minlength="6">
           </div>
+          <div class="form-group">
+            <label class="form-label">验证码</label>
+            <div style="display: flex; gap: 10px; align-items: center;">
+              <input type="text" class="form-input" v-model="registerForm.captcha" placeholder="请输入验证码" required style="flex: 1;">
+              <img v-if="registerCaptchaUrl" :src="registerCaptchaUrl" @click="refreshRegisterCaptcha" style="height: 44px; cursor: pointer; border-radius: 4px;" title="点击刷新验证码">
+            </div>
+          </div>
           <button type="submit" class="btn btn-primary">
             <i class="fas fa-user-plus"></i> 注册
           </button>
@@ -2590,7 +2604,7 @@
 
     <!-- 忘记密码模态框 -->
     <div v-if="showForgotPasswordModal" class="modal-overlay" @mousedown.self="handleModalMouseDown" @mouseup.self="handleModalMouseUp('showForgotPasswordModal')">
-      <div class="modal-content" @click.stop>
+      <div class="modal-content" @click.stop @focusin="!forgotPasswordCaptchaUrl && refreshForgotPasswordCaptcha()">
         <h3 style="margin-bottom: 20px;">忘记密码 - 邮箱重置</h3>
         <p style="color: var(--text-secondary); margin-bottom: 15px; font-size: 14px;">
           请输入注册邮箱，我们会发送重置链接到您的邮箱
@@ -2599,11 +2613,18 @@
           <label class="form-label">邮箱</label>
           <input type="email" class="form-input" v-model="forgotPasswordForm.email" placeholder="请输入注册邮箱" required>
         </div>
+        <div class="form-group">
+          <label class="form-label">验证码</label>
+          <div style="display: flex; gap: 10px; align-items: center;">
+            <input type="text" class="form-input" v-model="forgotPasswordForm.captcha" placeholder="请输入验证码" required style="flex: 1;">
+            <img v-if="forgotPasswordCaptchaUrl" :src="forgotPasswordCaptchaUrl" @click="refreshForgotPasswordCaptcha" style="height: 44px; cursor: pointer; border-radius: 4px;" title="点击刷新验证码">
+          </div>
+        </div>
         <div style="display: flex; gap: 10px; margin-top: 20px;">
           <button class="btn btn-primary" @click="requestPasswordReset" style="flex: 1;">
             <i class="fas fa-paper-plane"></i> 发送重置邮件
           </button>
-          <button class="btn btn-secondary" @click="showForgotPasswordModal = false; forgotPasswordForm = {email: ''}" style="flex: 1;">
+          <button class="btn btn-secondary" @click="showForgotPasswordModal = false; forgotPasswordForm = {email: '', captcha: ''}; forgotPasswordCaptchaUrl = ''" style="flex: 1;">
             <i class="fas fa-times"></i> 取消
           </button>
         </div>