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-relay/Cargo.toml

@@ -21,6 +21,7 @@ serde = { workspace = true }
 toml = "0.8"
 anyhow = "1"
 rustls = { version = "0.23", default-features = false, features = ["ring", "std"] }
+quinn = { workspace = true }
 
 [[bin]]
 name = "wzp-relay"