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/cli.rs

@@ -46,7 +46,7 @@ struct CliArgs {
     mnemonic: Option<String>,
     room: Option<String>,
     token: Option<String>,
-    metrics_file: Option<String>,
+    _metrics_file: Option<String>,
 }
 
 impl CliArgs {
@@ -220,7 +220,7 @@ fn parse_args() -> CliArgs {
         mnemonic,
         room,
         token,
-        metrics_file,
+        _metrics_file: metrics_file,
     }
 }