Add per-peer listener-side noise gate

Let a listener apply a noise gate to an individual peer's incoming
audio — "fix this person's noisy mic / background hum on my end" —
which is only possible because full-mesh P2P keeps every peer's stream
unmixed locally (server-mixed apps can't do per-listener per-peer DSP).

The DSP is the existing mic NoiseGate reused verbatim: it already
processes i16 frames at a fixed rate with hysteresis/attack/release/
hangover and takes the threshold per-frame. Wiring mirrors per-peer EQ:
- AppConfig.peer_gate map (threshold per peer id; absent/0 = off),
  persisted, never sent over the wire
- CoreCommand::SetPeerGate + Arc<Mutex<HashMap>> shared into the mixer
- a live HashMap<EndpointId, NoiseGate> in the mixer task, created
  lazily and dropped when disabled (no rebuild needed — threshold is
  passed per frame)
- Gate row (threshold slider, "Off" at zero) in each participant card
  next to Vol/Pan/EQ, persisting on release

The gate runs on the raw decoded frame: after the clean multitrack stem
tap (recordings stay ungated) but before volume/EQ, so the threshold
tracks the peer's true signal level regardless of our volume setting.
Same 0..METER_MAX scale as the mic gate.

+2 unit tests (config helper); +1 config back-compat assertion.

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

src/app/mod.rs

@@ -208,6 +208,7 @@ pub enum AppMessage {
     ClearHotkey(HotkeyAction),
     PeerVolumeChanged(EndpointId, f32),
     PeerPanChanged(EndpointId, f32),
+    PeerGateChanged(EndpointId, f32),
     PeerEqChanged(EndpointId, EqBand, f32),
     /// Toggle local mute of a peer (silence them just for us).
     TogglePeerMute(EndpointId),
@@ -478,6 +479,11 @@ impl Default for AppState {
                 let _ = controller.send(CoreCommand::SetPeerVolume(id, *volume));
             }
         }
+        for (peer, threshold) in &config.peer_gate {
+            if let Ok(id) = peer.parse::<EndpointId>() {
+                let _ = controller.send(CoreCommand::SetPeerGate(id, *threshold));
+            }
+        }
         let pixelpass_available =
             crate::screenshare::is_available(config.pixelpass_path.as_deref());
         let all_devices = enumerate_audio_devices();
@@ -761,6 +767,20 @@ fn set_peer_volume_config(config: &mut AppConfig, id: EndpointId, volume: f32) -
     volume
 }
 
+/// Store the per-peer listener noise-gate threshold, clamped to the slider
+/// range. `0.0` means the gate is off, so an at-zero entry is removed rather
+/// than stored. Returns the clamped value.
+fn set_peer_gate_config(config: &mut AppConfig, id: EndpointId, threshold: f32) -> f32 {
+    let threshold = threshold.clamp(0.0, METER_MAX);
+    let key = id.to_string();
+    if threshold <= 0.0 {
+        config.peer_gate.remove(&key);
+    } else {
+        config.peer_gate.insert(key, threshold);
+    }
+    threshold
+}
+
 fn peer_eq_settings(config: &AppConfig, id: &EndpointId) -> EqSettings {
     config
         .peer_eq
@@ -1062,6 +1082,10 @@ fn update(state: &mut AppState, message: AppMessage) -> Task<AppMessage> {
             let pan = set_peer_pan_config(&mut state.config, id, pan);
             let _ = state.controller.send(CoreCommand::SetPeerPan(id, pan));
         }
+        AppMessage::PeerGateChanged(id, threshold) => {
+            let threshold = set_peer_gate_config(&mut state.config, id, threshold);
+            let _ = state.controller.send(CoreCommand::SetPeerGate(id, threshold));
+        }
         AppMessage::PeerEqChanged(id, band, gain_db) => {
             let settings = set_peer_eq_config(&mut state.config, id, band, gain_db);
             let _ = state.controller.send(CoreCommand::SetPeerEq(id, settings));
@@ -3395,6 +3419,28 @@ fn view(state: &AppState) -> Element<'_, AppMessage> {
                 .align_y(iced::alignment::Vertical::Center),
             );
 
+            // Peer noise gate: suppress this peer's background noise on our end.
+            // Threshold is normalized RMS on the same 0..METER_MAX scale as the
+            // mic gate; 0 = off.
+            let current_gate = state.config.peer_gate.get(&peer_key).copied().unwrap_or(0.0);
+            let gate_label = if current_gate <= 0.0 {
+                "Off".to_string()
+            } else {
+                format!("{:.0}%", (current_gate / METER_MAX * 100.0).clamp(0.0, 100.0))
+            };
+            card_content = card_content.push(
+                row![
+                    text("Gate:").size(12).color(color_subtext),
+                    container(text(gate_label).size(11).color(color_subtext))
+                        .width(iced::Length::Fixed(58.0)),
+                    slider(0.0..=METER_MAX, current_gate, move |v| AppMessage::PeerGateChanged(peer_id_clone, v))
+                        .step(0.001)
+                        .on_release(AppMessage::PersistConfig),
+                ]
+                .spacing(8)
+                .align_y(iced::alignment::Vertical::Center),
+            );
+
             let eq = peer_eq_settings(&state.config, peer_id);
             let eq_row = |label: &'static str, band: EqBand, value: f32| -> Element<'_, AppMessage> {
                 row![
@@ -4924,10 +4970,28 @@ impl Program<AppMessage> for Icon {
 mod tests {
     use super::{
         format_duration, initial_window_position, reconnect_attempt_chime, reconnected_chime,
-        set_peer_volume_config, AppConfig, GateMeter, METER_MAX,
+        set_peer_gate_config, set_peer_volume_config, AppConfig, GateMeter, METER_MAX,
     };
     use iroh::SecretKey;
 
+    #[test]
+    fn peer_gate_persists_when_on_and_clears_when_off() {
+        let mut config = AppConfig::default();
+        let id = SecretKey::generate().public();
+
+        // A positive threshold is stored, clamped to the slider's METER_MAX ceiling.
+        let stored = set_peer_gate_config(&mut config, id, 0.05);
+        assert_eq!(stored, 0.05);
+        assert_eq!(config.peer_gate.get(&id.to_string()).copied(), Some(0.05));
+        assert_eq!(set_peer_gate_config(&mut config, id, 99.0), METER_MAX);
+
+        // Zero (or negative) means "gate off" — the entry is removed so the
+        // config doesn't carry a disabled gate.
+        let stored = set_peer_gate_config(&mut config, id, 0.0);
+        assert_eq!(stored, 0.0);
+        assert!(!config.peer_gate.contains_key(&id.to_string()));
+    }
+
     #[test]
     fn peer_volume_persists_non_unity_and_clears_at_unity() {
         let mut config = AppConfig::default();