fix: KCP/UDP resilience to ICMP resets; optional KCP congestion control

- treat ICMP-driven UDP socket errors (WSAECONNRESET 10054 on Windows,
  ECONNREFUSED on Linux) as packet loss in punch_udp and the KCP pump
  instead of tearing the session down; KCP retransmits through them and a
  truly dead link is still reaped by the pong/app-level timeouts
- resolve STUN hostnames via tokio::net::lookup_host so DNS never blocks a
  runtime worker; fix the inverted non-IPv4 error message
- add enable-kcp-congestion-control option (default on): switch the turbo
  profile to nc=0 so brief loss on constrained links no longer spirals into
  stalls; sender-side only, no wire negotiation
- pin kcp-sys to the rustdesk-patches branch: upstream main lost the
  RustDesk patches on the EasyTier sync, and this branch also wires
  set_kcp_config_factory into connection setup, making the option effective

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Cargo.toml

@@ -83,7 +83,7 @@ fon = "0.6"
 shutdown_hooks = "0.1"
 totp-rs = { version = "5.4", default-features = false, features = ["gen_secret", "otpauth"] }
 stunclient = "0.4"
-kcp-sys= { git = "https://github.com/rustdesk-org/kcp-sys"}
+kcp-sys= { git = "https://github.com/rustdesk-org/kcp-sys", branch = "rustdesk-patches" }
 reqwest = { version = "0.12", features = ["blocking", "socks", "json", "native-tls", "rustls-tls", "rustls-tls-native-roots", "gzip", "zstd"], default-features=false }
 
 [target.'cfg(not(target_os = "linux"))'.dependencies]