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

internal/dhcp/server.go

@@ -700,3 +700,26 @@ func newBroadcastUDPConn(host string, port int) (*net.UDPConn, error) {
 
 	return udpConn, nil
 }
+
+// EvictClient removes a client's lease, forcing them to get a new IP
+func (s *Server) EvictClient(mac string) error {
+	s.leaseMutex.Lock()
+	defer s.leaseMutex.Unlock()
+
+	lease, exists := s.leases[mac]
+	if !exists {
+		return fmt.Errorf("lease not found for MAC %s", mac)
+	}
+
+	// Remove from in-memory lease map
+	delete(s.leases, mac)
+	delete(s.usedIPs, lease.IP)
+
+	// Delete from database
+	if err := s.db.Where("mac = ?", mac).Delete(&db.DHCPLease{}).Error; err != nil {
+		return fmt.Errorf("failed to delete lease from database: %v", err)
+	}
+
+	log.Printf("DHCP: Evicted client %s (%s)", mac, lease.IP)
+	return nil
+}