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>

Cargo.lock

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+
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@@ -2275,6 +2281,7 @@ dependencies = [
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  "iced_futures",
  "log",
+ "lyon_path",
  "raw-window-handle",
  "rustc-hash 2.1.2",
  "thiserror 2.0.18",
@@ -2349,6 +2356,7 @@ dependencies = [
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  "iced_graphics",
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+ "lyon",
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  "thiserror 2.0.18",
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+ "euclid",
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