feat: 添加用户注册功能前端页面

- 新增 Signup.vue 注册页面
- 路由添加 /signup 路由
- Login.vue 注册链接跳转到注册页

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/views/Login.vue

@@ -149,7 +149,7 @@ const handleLogin = () => {
       <!-- 注册链接 -->
       <p class="text-center mt-6 text-gray-400">
         Don't have an account?
-        <a href="#" class="text-primary-orange hover:text-orange-400 font-medium transition-colors">Sign up</a>
+        <a @click="router.push('/signup')" class="text-primary-orange hover:text-orange-400 font-medium transition-colors cursor-pointer">Sign up</a>
       </p>
     </div>
   </div>