Session establishment race condition

[rafabd1]

In handle_incoming() (session.rs:267-269), if a peer tries to connect while one is already active:

if state.session.lock().await.is_some() {
    return Ok(());  // Silent return, connection not acknowledged
}

This causes:

1. Connection hangs: The initiator sends HANDSHAKE_INIT and waits for ACK indefinitely
2. No protocol error: The peer never sends back an error, just closes the stream
3. User confusion: Both users think they're waiting for a response

Scenario:

• Alice and Bob already have a session
• Charlie tries to connect to Alice at the same time Bob sends a message
• Charlie's HANDSHAKE_INIT is dropped, Charlie hangs
• Alice sees no indication of the failed connection attempt

Proposed solution:

• Send an explicit protocol error (e.g., type="err", code="session_active")
• Or: Support multiple concurrent sessions (complex, but aligns with group chat)
• Track and log failed connection attempts for user visibility

[rafabd1]

• Or: Support multiple concurrent sessions (complex, but aligns with group chat)

Multiple sessions still won't be implemented.