fix UDP forwarding

[diyism]

Changes

1. Added ForwardUDPV3 (tsproxy/port_forward_udp_v3.go)
   • Uses Listen() + Accept() for proper tsnet UDP handling
   • Simplified connection-oriented approach with io.Copy
2. Added listenUDPAsListener helper (tsproxy/port_forward_udp_fixed.go)
   • Abstracts the difference between Tailscale and regular UDP listening
3. Updated main.go
   • Changed UDP forwarding to use ForwardUDPV3 by default
4. Fixed timeout bug in original ForwardUDP
   • Changed time.Duration(t.udpTimeout) to time.Duration(t.udpTimeout) * time.Second

[ge9]

Thank you for the pull request!
The current ForwardUDP is AI-generated, so it's okay to totally replace it with your PR.
I have a question, what is the functional difference between ForwardUDPFixed and ForwardUDPV3?
We can choose either one if one is better than the other, or add a new option to switch between them if there is a trade-off.

[diyism]

My fault, I should get rid of ForwardUDPFixed():

main.go
  └─> ForwardUDPV3()  ✓ used  (from port_forward_udp_v3.go)
        └─> listenUDPAsListener()  ✓ used (from port_forward_udp_fixed.go)
ForwardUDPFixed()  ✗ unused