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/core/messages.rs

@@ -26,6 +26,10 @@ pub enum CoreCommand {
     SetPeerEq(EndpointId, crate::audio::eq::EqSettings),
     /// Listener-side per-peer pan. Local only; never leaves this app instance.
     SetPeerPan(EndpointId, f32),
+    /// Listener-side per-peer noise gate threshold (normalized RMS, `0.0` = off).
+    /// Applies the same smooth gate as the mic path to a peer's incoming audio,
+    /// to suppress their background noise on our end. Local only.
+    SetPeerGate(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),