feat: add VAD threshold controls and smooth gate ramp

Replace the hard 0/1 VAD gate with a 20ms ramp in the worklet to prevent
clicks on open/close transitions. Expose positive and negative speech
probability thresholds as user-adjustable settings (defaults 0.5/0.35).
Sliders with restore-defaults button added to the VAD section of the
audio settings tab.

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

src/state/CallViewModel/localMember/Publisher.ts

@@ -42,6 +42,8 @@ import {
   transientThreshold,
   transientRelease,
   vadEnabled,
+  vadPositiveThreshold,
+  vadNegativeThreshold,
 } from "../../../settings/settings.ts";
 import {
   type NoiseGateParams,
@@ -467,7 +469,10 @@ export class Publisher {
         return;
       }
       const stream = new MediaStream([rawTrack]);
-      vadGate = new SileroVADGate(stream, ctx);
+      vadGate = new SileroVADGate(stream, ctx, {
+        positiveThreshold: vadPositiveThreshold.getValue(),
+        negativeThreshold: vadNegativeThreshold.getValue(),
+      });
       vadGate.onOpen  = (): void => transformer?.setVADOpen(true);
       vadGate.onClose = (): void => transformer?.setVADOpen(false);
       vadGate.start().catch((e: unknown) => {
@@ -525,6 +530,13 @@ export class Publisher {
         }
       });
 
+    // Push VAD threshold changes to the live gate without recreating it.
+    combineLatest([vadPositiveThreshold.value$, vadNegativeThreshold.value$])
+      .pipe(scope.bind())
+      .subscribe(([positiveThreshold, negativeThreshold]) => {
+        vadGate?.updateOptions({ positiveThreshold, negativeThreshold });
+      });
+
     // Push param changes to the live worklet without recreating the processor.
     combineLatest([
       noiseGateThreshold.value$,