fix(gateway): show iface alias in help

The MVP test guide tells operators that --iface is accepted as a shorter alias
for --interface, but Clap hid the alias from lanparty-gateway --help. Make the
alias visible so the binary's self-documentation matches the handoff guide.

This does not change parsing behavior; --iface was already accepted and covered
by the existing gateway parser test.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-gateway accepts_iface_alias_for_gateway_interface
- cargo run -p lanparty-gateway -- --help
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
- git diff --cached --check

Refs: MVP gateway test handoff

crates/lanparty-gateway/src/lib.rs

@@ -87,7 +87,7 @@ pub struct GatewayArgs {
     room: RoomCode,
 
     /// Wired LAN interface that will later be opened with AF_PACKET.
-    #[arg(long, alias = "iface")]
+    #[arg(long, visible_alias = "iface")]
     interface: String,
 
     /// Gateway's advertised QUIC datagram budget before relay clamping.