feat: replace ScriptProcessorNode with AudioWorkletNode for transmit

- Create audio-worklet.js with MicrophoneProcessor for audio capture
- Implement noise gate and RMS calculation in worklet
- Send PCM data via MessagePort to main thread
- Update startStreaming to use AudioWorkletNode instead of deprecated ScriptProcessorNode
- Remove WebCodecs decoder complexity from listen
- Keep simple PCM playback for listen feature

public/audio-worklet.js

@@ -0,0 +1,42 @@
+class MicrophoneProcessor extends AudioWorkletProcessor {
+  constructor() {
+    super();
+    this.noiseGateThreshold = 0.01;
+    this.noiseGateHoldFrames = 3;
+    this.noiseGateHold = 0;
+  }
+
+  process(inputs, outputs, parameters) {
+    const input = inputs[0];
+    if (!input || input.length === 0) return true;
+
+    const inputData = input[0];
+    const output = outputs[0];
+    if (output && output.length > 0) {
+      output[0].set(inputData);
+    }
+
+    let sum = 0;
+    for (let i = 0; i < inputData.length; i++) {
+      sum += inputData[i] * inputData[i];
+    }
+    const rms = Math.sqrt(sum / inputData.length);
+
+    if (rms < this.noiseGateThreshold && this.noiseGateHold <= 0) {
+      this.port.postMessage({ type: 'audio', rms: 0, data: null });
+      return true;
+    }
+
+    this.noiseGateHold = rms >= this.noiseGateThreshold ? this.noiseGateHoldFrames : this.noiseGateHold - 1;
+
+    const pcm = new Int16Array(inputData.length);
+    for (let i = 0; i < inputData.length; i++) {
+      pcm[i] = Math.max(-1, Math.min(1, inputData[i])) * 32767;
+    }
+
+    this.port.postMessage({ type: 'audio', rms, data: pcm.buffer }, [pcm.buffer]);
+    return true;
+  }
+}
+
+registerProcessor('microphone-processor', MicrophoneProcessor);