feat: settings page with persistence, client alias in handshake, fix null fingerprints

- Add SettingsScreen with identity (alias, key backup/restore), audio defaults,
  server management, network prefs, and default room
- SettingsRepository persists all settings via SharedPreferences
- Auto-generate random display names on first launch (e.g. "Swift Wolf")
- Thread alias through CallOffer → relay handshake → RoomUpdate broadcast
- Derive caller fingerprint from identity key in relay handshake (fixes null
  fingerprints when --auth-url is not set)
- Persist identity seed for stable fingerprints across reconnects
- Add alias field to SignalMessage::CallOffer (serde default for backward compat)

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

crates/wzp-client/src/cli.rs

@@ -287,6 +287,7 @@ async fn main() -> anyhow::Result<()> {
     let _crypto_session = wzp_client::handshake::perform_handshake(
         &*transport,
         &seed.0,
+        None, // alias — desktop client doesn't set one yet
     ).await?;
     info!("crypto handshake complete");