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/config.rs

@@ -259,6 +259,11 @@ pub struct AppConfig {
     /// string. Local preference only; never sent to peers. Absent entry = unity.
     #[serde(default)]
     pub peer_volume: HashMap<String, f32>,
+    /// Per-peer listener-side noise-gate threshold (normalized RMS, `0.0` = off),
+    /// keyed by peer node id string. Local preference only; never sent to peers.
+    /// Absent entry = gate disabled (pass-through).
+    #[serde(default)]
+    pub peer_gate: HashMap<String, f32>,
     /// Focused app-local keyboard shortcuts.
     #[serde(default)]
     pub hotkeys: crate::hotkeys::HotkeyMap,
@@ -322,6 +327,7 @@ impl Default for AppConfig {
             peer_eq: HashMap::new(),
             peer_pan: HashMap::new(),
             peer_volume: HashMap::new(),
+            peer_gate: HashMap::new(),
             hotkeys: crate::hotkeys::HotkeyMap::default(),
             window_width: default_window_width(),
             window_height: default_window_height(),
@@ -463,6 +469,7 @@ mod tests {
         assert!(deserialized.peer_eq.is_empty());
         assert!(deserialized.peer_pan.is_empty());
         assert!(deserialized.peer_volume.is_empty());
+        assert!(deserialized.peer_gate.is_empty());
         assert_eq!(
             crate::hotkeys::format_binding(
                 deserialized