feat: add Silero VAD toggle to audio pipeline

Integrates @ricky0123/vad-web's MicVAD as an optional voice activity detector
alongside the noise gate. When enabled, the Silero ONNX model classifies each
audio frame as speech or silence; silence frames mute the worklet's output via
a new VAD gate message. VAD is wired into Publisher.ts alongside the existing
noise gate transformer. Vite is configured to copy the worklet bundle, ONNX
model, and ORT WASM files to /vad/ so they're reachable at runtime.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/settings/settings.ts

@@ -145,6 +145,8 @@ export const noiseGateHold = new Setting<number>("noise-gate-hold", 200);
 // Time in ms for the gate to fully close after hold expires
 export const noiseGateRelease = new Setting<number>("noise-gate-release", 150);
 
+export const vadEnabled = new Setting<boolean>("vad-enabled", false);
+
 export const transientSuppressorEnabled = new Setting<boolean>(
   "transient-suppressor-enabled",
   false,