feat: add configurable TCP flags for GFK backend (#21)

Add GFK_TCP_FLAGS setting to allow customizing TCP flags used in
violated TCP packets. This addresses cases where different flag
combinations (e.g. 'S', 'RA') may work better for certain networks.

Changes:
- Add tcp_flags parameter to vio_server.py and vio_client.py
- Add GFK_TCP_FLAGS to settings save/load with validation
- Add TCP flags menu option in change config for both server/client
- Default remains 'AP' for backwards compatibility

Validation: Only uppercase TCP flags allowed (F,S,R,P,A,U,E,C)

gfk/server/vio_server.py

@@ -18,6 +18,7 @@ vio_tcp_server_port = parameters.vio_tcp_server_port
 vio_udp_server_port = parameters.vio_udp_server_port
 quic_local_ip = parameters.quic_local_ip
 quic_server_port = parameters.quic_server_port
+tcp_flags = getattr(parameters, 'tcp_flags', 'AP')
 
 
 
@@ -89,7 +90,7 @@ async def forward_vio_to_quic(qu1, transport):
 
 
 
-basepkt = IP() / TCP(sport=vio_tcp_server_port, seq=1, flags="AP", ack=0, options=tcp_options) / Raw(load=b"")
+basepkt = IP() / TCP(sport=vio_tcp_server_port, seq=1, flags=tcp_flags, ack=0, options=tcp_options) / Raw(load=b"")
 
 skt = conf.L3socket()