feat: add TCP Server, UDP Server, and COM port forwarding modes

Extend Mode constants (3=TCPServer, 4=UDPServer, 5=COMPort) with
explicit values. Refactor Target to support multiple connection types
(listener+conns map, packetConn, serialPort). Add acceptLoop,
readLoopPacket, readLoopSerial. Mode-aware Broadcast dispatches to
all accepted conns (TCP-S), known remotes (UDP-S), or serial port.
Update flag help, command completions, and TUI panel hints.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/command/dispatcher.go

@@ -99,8 +99,8 @@ func (d *Dispatcher) registerAll() {
 
 	d.register(RuntimeCommand{
 		Name:        ".forward",
-		Usage:       ".forward <list|add|remove|enable|disable|update|stats>",
-		Description: "manage forwarding at runtime",
+		Usage:       ".forward <list|add|remove|enable|disable|update>",
+		Description: "manage forwarding (tcp/udp/tcp-s/udp-s/com)",
 		Handler:     d.handleForwardCommand,
 		Completer:   completeForward,
 	})