feat: add VAD mode setting — standard vs aggressive latency

Standard: 5 ms open / 20 ms close ramp (comfortable feel)
Aggressive: 1 ms open / 5 ms close ramp (lowest possible latency)

The mode is surfaced as a radio selector in Settings → Audio → Voice
activity detection, visible while VAD is enabled. Wired through
NoiseGateParams.vadAggressive → worklet updateParams.

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

src/livekit/NoiseGateTransformer.ts

@@ -23,6 +23,7 @@ export interface NoiseGateParams {
   vadEnabled: boolean;
   vadPositiveThreshold: number; // open gate when isSpeech prob >= this (0–1)
   vadNegativeThreshold: number; // close gate when isSpeech prob < this (0–1)
+  vadAggressive: boolean;       // true: 1 ms open / 5 ms close ramp; false: 5 ms / 20 ms
 }
 
 /**