fix: 编辑帖子时验证token有效性

- checkLogin时调用API验证token
- saveEdit/submitReply/likePost函数增加token检查
- token过期时自动跳转登录页

frontend/post.html

@@ -113,9 +113,24 @@
             const user = localStorage.getItem('user');
             
             if (token && user) {
-                currentUser = JSON.parse(user);
-                document.getElementById('replyForm').classList.remove('hidden');
-                document.getElementById('loginHint').classList.add('hidden');
+                // 验证 token 是否有效
+                try {
+                    const res = await fetch('/api/user', {
+                        headers: { 'Authorization': 'Bearer ' + token }
+                    });
+                    if (res.ok) {
+                        currentUser = JSON.parse(user);
+                        document.getElementById('replyForm').classList.remove('hidden');
+                        document.getElementById('loginHint').classList.add('hidden');
+                    } else {
+                        // token 无效，清除
+                        localStorage.removeItem('token');
+                        localStorage.removeItem('user');
+                        currentUser = null;
+                    }
+                } catch (e) {
+                    console.error('验证登录失败', e);
+                }
             }
         }
 
@@ -274,6 +289,11 @@
         }
 
         async function saveEdit() {
+            if (!currentUser) {
+                alert('请先登录');
+                return;
+            }
+            
             const title = document.getElementById('editTitle').value.trim();
             const content = document.getElementById('editContent').value.trim();
             const tagsStr = document.getElementById('editTags').value.trim();
@@ -288,11 +308,18 @@
                 return;
             }
             
+            const token = localStorage.getItem('token');
+            if (!token) {
+                alert('登录已过期，请重新登录');
+                window.location.href = '/login';
+                return;
+            }
+            
             try {
                 const res = await fetch('/api/posts/' + currentPostId + '/edit', {
                     method: 'POST',
                     headers: {
-                        'Authorization': 'Bearer ' + localStorage.getItem('token'),
+                        'Authorization': 'Bearer ' + token,
                         'Content-Type': 'application/json'
                     },
                     body: JSON.stringify({ title, content, tags })
@@ -324,11 +351,18 @@
                 return;
             }
 
+            const token = localStorage.getItem('token');
+            if (!token) {
+                alert('登录已过期，请重新登录');
+                window.location.href = '/login';
+                return;
+            }
+
             try {
                 const res = await fetch('/api/posts/' + currentPostId + '/reply', {
                     method: 'POST',
                     headers: {
-                        'Authorization': 'Bearer ' + localStorage.getItem('token'),
+                        'Authorization': 'Bearer ' + token,
                         'Content-Type': 'application/json'
                     },
                     body: JSON.stringify({ content })
@@ -352,11 +386,18 @@
                 return;
             }
 
+            const token = localStorage.getItem('token');
+            if (!token) {
+                alert('登录已过期，请重新登录');
+                window.location.href = '/login';
+                return;
+            }
+
             try {
                 const res = await fetch('/api/posts/' + currentPostId + '/like', {
                     method: 'POST',
                     headers: {
-                        'Authorization': 'Bearer ' + localStorage.getItem('token')
+                        'Authorization': 'Bearer ' + token
                     }
                 });
                 const data = await res.json();