feat(net): accept relay hostnames

PLAN.md describes the first client flow as entering a relay domain and room
code, but the client and gateway CLIs only accepted socket-address literals.
Add a small shared RelayEndpoint parser so bare hosts default to UDP/443 while
IP literals and explicit host:port values stay supported.

The runtime configs still store resolved SocketAddr values. That keeps the
Windows route-pinning path on a concrete relay IP before TAP activation while
avoiding duplicated endpoint grammar between client and gateway. The relay
listen config reuses the same default port constant so UDP/443 has one source.

README examples now use lanparty-relay.local and document the shared endpoint
syntax.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-net
- cargo test -p lanparty-client-win \
  accepts_relay_domain_with_default_port -- --nocapture
- cargo test -p lanparty-gateway \
  accepts_iface_alias_for_gateway_interface -- --nocapture
- cargo test -p lanparty-net -p lanparty-client-win -p lanparty-gateway
- cargo clippy -p lanparty-net -p lanparty-client-win -p lanparty-gateway \
  --all-targets -- -D warnings
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check

Refs: PLAN.md

crates/lanparty-relay/Cargo.toml

@@ -8,6 +8,7 @@ anyhow.workspace = true
 bytes.workspace = true
 clap.workspace = true
 lanparty-ctrl = { path = "../lanparty-ctrl" }
+lanparty-net = { path = "../lanparty-net" }
 lanparty-obs = { path = "../lanparty-obs" }
 lanparty-proto = { path = "../lanparty-proto" }
 quinn.workspace = true

crates/lanparty-relay/src/config.rs

@@ -6,12 +6,11 @@ use std::{
 };
 
 use clap::Parser;
+pub use lanparty_net::DEFAULT_RELAY_PORT;
 use thiserror::Error;
 
 use crate::DEFAULT_MAX_CLIENTS_PER_ROOM;
 
-pub const DEFAULT_RELAY_PORT: u16 = 443;
-
 #[derive(Debug, Clone, PartialEq, Eq, Parser)]
 #[command(
     name = "lanparty-relay",