feat(标签管理): 添加标签删除功能并优化界面样式

- 在后端添加删除标签的API端点，包含权限检查和级联删除
- 在前端添加标签删除按钮和确认对话框
- 优化标签管理界面的样式和交互体验
- 改进导航标签栏的设计

backend/server.js

@@ -296,6 +296,32 @@ async function startServer() {
     }
   })
 
+  // NEW: DELETE a tag
+  app.delete('/api/managers/tags/:id', authenticateJWT, async (req, res) => {
+    try {
+      const { id } = req.params
+
+      // Optional: Check if the user is a manager before allowing deletion
+      if (req.user.role !== 'manager') {
+        return res.status(403).json({ message: 'Forbidden: Only managers can delete tags.' })
+      }
+
+      // Delete the tag from the 'tags' table.
+      // If 'worker_tags' table has ON DELETE CASCADE for tag_id,
+      // related entries in 'worker_tags' will automatically be removed.
+      const [result] = await db.execute('DELETE FROM tags WHERE id = ?', [id])
+
+      if (result.affectedRows === 0) {
+        return res.status(404).json({ message: 'Tag not found.' })
+      }
+
+      res.status(204).send() // 204 No Content for successful deletion
+    } catch (error) {
+      console.error('Delete tag error:', error)
+      res.status(500).json({ message: 'Database error deleting tag.' })
+    }
+  })
+
   // POST to assign a tag to a worker
   app.post('/api/managers/workers/:workerId/tags', authenticateJWT, async (req, res) => {
     try {
@@ -484,7 +510,6 @@ async function startServer() {
         return res.status(409).json({ message: `Worker is already clocked ${status}.` })
       }
       // --- THIS IS THE FIX ---
-      // Sanitize the timestamp from "YYYY-MM-DDTHH:mm" to "YYYY-MM-DD HH:mm"
       const sanitizedTimestamp = timestamp.replace('T', ' ')
 
       await db.execute(