feat(room): per-peer VU meters, own mic meter, local mute, call info bar

Enriches the in-room screen:
- Per-peer VU meters: a live level bar per peer card (reuses the per-peer
  audio_levels stream), green while speaking, dim when idle/locally-muted.
- Your own mic meter on the self-card (reuses the in-call MicLevel), green
  when transmitting, grey when muted or PTT-inactive.
- Per-peer local mute (🔊/🔇): silences a peer for you only — decoded so
  their VU still moves, but not mixed. New CoreCommand::SetPeerMuted + a
  locally_muted set in the core/mixer, distinct from per-peer volume.
- Header call-info: participant count + a live m:ss / h:mm:ss call timer
  (dependency-free — rides the in-call event stream rather than a tick sub).

format_duration unit-tested. Build + clippy clean, 70 lib tests.
Field-verified on a real desktop<->dopedart call.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/core/messages.rs

@@ -11,6 +11,9 @@ pub enum CoreCommand {
     SetPttMode(bool),
     SetPttActive(bool),
     SetPeerVolume(EndpointId, f32),
+    /// Locally mute/unmute a peer: when muted, their audio is decoded (so levels
+    /// still show) but not mixed into our output.
+    SetPeerMuted(EndpointId, bool),
     SetNoiseGateThreshold(f32),
     /// App-internal capture gain (mic), applied before the gate/encode. 1.0 = unity.
     SetInputVolume(f32),