fix: eliminate all compiler warnings across client, relay, web

- Remove unused imports in featherchat.rs (tracing, QualityProfile)
- Remove unused comfort_noise field from CallEncoder (cn_level is used instead)
- Prefix unused _metrics_file in CliArgs
- Prefix unused _addr in Participant
- Remove unused RoomSlot struct and rooms field from web AppState
- Remove unused HashMap import from web main

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

crates/wzp-client/src/featherchat.rs

@@ -11,10 +11,7 @@
 //! 5. Connects QUIC to relay for media
 
 use serde::{Deserialize, Serialize};
-use tracing::{error, info};
-
 use wzp_proto::packet::SignalMessage;
-use wzp_proto::QualityProfile;
 
 /// featherChat CallSignal types (mirrors warzone-protocol::message::CallSignalType).
 #[derive(Clone, Debug, Serialize, Deserialize)]