feat: multi-party rooms (SFU) + push-to-talk radio mode

Room-based SFU relay:
- Clients join named rooms (room name from QUIC SNI)
- Each participant's packets forwarded to all others (no mixing)
- Multiple rooms run concurrently on one relay
- Web bridge passes room name from URL path to relay

Push-to-talk (radio mode):
- Toggle "Radio mode" checkbox after connecting
- Hold PTT button or spacebar to transmit
- Release to mute mic (receive-only)
- Works on desktop (spacebar) and mobile (touch)

URL routing:
- /myroom → joins room "myroom"
- Room name input field as fallback

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-web/src/main.rs

@@ -155,8 +155,10 @@ async fn handle_ws(socket: WebSocket, room: String, state: AppState) {
         Err(e) => { error!("create endpoint: {e}"); return; }
     };
 
+    // Pass room name as QUIC SNI so the relay knows which room to join
+    let sni = if room.is_empty() { "default" } else { &room };
     let connection =
-        match wzp_transport::connect(&endpoint, relay_addr, "localhost", client_config).await {
+        match wzp_transport::connect(&endpoint, relay_addr, sni, client_config).await {
             Ok(c) => c,
             Err(e) => { error!("connect to relay: {e}"); return; }
         };