fix(relay): close failed accepted handshakes

After a peer is registered, later accepted-handshake failures clean up room and
session state. If the welcome was already sent, the joining peer could otherwise
keep a live QUIC connection briefly while the relay has removed it from room
membership.

Close the connection after accepted-handshake cleanup so the peer learns the
handshake failed instead of continuing with stale local state.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-relay
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
- git diff --cached --check

Refs: MVP lifecycle ordering

crates/lanparty-relay/src/server.rs

@@ -317,6 +317,7 @@ async fn accept_control_handshake(
                         accepted.peer.role() == Role::Client,
                     )
                     .await?;
+                    close_failed_accepted_handshake(connection);
 
                     return Err(error);
                 }
@@ -332,6 +333,7 @@ async fn accept_control_handshake(
                         accepted.peer.role() == Role::Client,
                     )
                     .await?;
+                    close_failed_accepted_handshake(connection);
 
                     return Err(error);
                 }
@@ -342,6 +344,10 @@ async fn accept_control_handshake(
     Ok(Some(accepted))
 }
 
+fn close_failed_accepted_handshake(connection: &quinn::Connection) {
+    connection.close(0_u32.into(), b"relay accepted handshake failed");
+}
+
 const fn accepted_handshake_steps(role: Role) -> &'static [AcceptedHandshakeStep] {
     match role {
         Role::Client => CLIENT_HANDSHAKE_STEPS,