feat(audio): live mic meter with draggable noise gate

Adds a mic input meter to Settings for gate calibration, replacing the
blind noise-gate slider with a unified Discord/OBS-style control: the bar
shows the live mic level and a draggable handle sets the gate threshold on
the same axis. Fill is green above the gate (transmitting), dim below it
(muted), with a live status word; the handle is bright red with a dark
edge so it stays legible when the green level sweeps past it.

Two level sources:
- In-call: the capture thread peak-holds the raw (pre-gate, pre-mute)
  frame level and emits UiEvent::MicLevel ~10/sec.
- Off-call: a "Test mic" toggle runs CoreCommand::SetMicMonitor, spinning
  up a standalone capture-only stream feeding run_mic_monitor. It shares
  the backend's single capture stream, so Join tears it down first and
  leaving Settings releases it; ignored while a session is active.

The gate handle drags live via NoiseGateDragging (no disk write per pixel)
and persists once on release via NoiseGateChanged. Meter axis is 0..0.3 so
a normal voice doesn't peg. Enables the iced "canvas" feature for the
custom GateMeter widget.

Build + clippy clean, tests pass. Field-verified on desktop.

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

src/core/messages.rs

@@ -12,6 +12,10 @@ pub enum CoreCommand {
     SetPttActive(bool),
     SetPeerVolume(EndpointId, f32),
     SetNoiseGateThreshold(f32),
+    /// Start/stop a standalone capture-only stream that reports the raw mic
+    /// level via [`UiEvent::MicLevel`], for gate calibration outside a call.
+    /// Ignored while a room session is active (the in-call meter covers that).
+    SetMicMonitor { enabled: bool, input_device: Option<String> },
     /// Set the relay/discovery posture. Takes effect on the next room join,
     /// since the endpoint is (re)built then.
     SetNetworkMode(NetworkMode),
@@ -30,5 +34,8 @@ pub enum UiEvent {
     /// Audio link to a peer is up and carrying audio.
     PeerConnected { id: EndpointId },
     AudioLevels(Vec<(EndpointId, f32)>),
+    /// Raw (pre-gate, pre-mute) normalized RMS of the local mic, `0.0..=1.0`,
+    /// for the settings level meter. Throttled to ~10/sec.
+    MicLevel(f32),
     Error(String),
 }