Add client eviction feature

- Add EvictClient method to DHCP Server
- Add /api/dhcp/leases/evict endpoint to force client IP release
- Add 'Evict' button in Web UI for online clients
- Update table layout to include Action column
- Evicted client will be forced to get a new IP on next DHCP request

web/templates/index.html

@@ -182,11 +182,12 @@
                             <th>租约剩余</th>
                             <th>过期时间</th>
                             <th>状态</th>
+                            <th>操作</th>
                         </tr>
                     </thead>
                     <tbody>
                         <tr>
-                            <td colspan="6" style="text-align:center;color:#999;">暂无客户端</td>
+                            <td colspan="7" style="text-align:center;color:#999;">暂无客户端</td>
                         </tr>
                     </tbody>
                 </table>