feat: room-based calls + AudioWorklet for capture and playback

Rooms:
- URL-based: open /myroom to join a room
- Two clients in same room get bridged through relay
- Input field for room name, also supports URL path and hash
- Each room creates independent relay connections

AudioWorklet (replaces deprecated ScriptProcessorNode):
- capture-processor.js: accumulates mic samples, sends 960-sample frames
- playback-processor.js: pull-based output with 200ms buffer cap
- Falls back to ScriptProcessor if AudioWorklet unavailable
- Eliminates drift: worklet runs on audio thread, not main thread

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-web/static/playback-processor.js

@@ -0,0 +1,45 @@
+// AudioWorklet processor for playing received audio.
+// Receives PCM samples from the main thread and outputs them.
+
+class PlaybackProcessor extends AudioWorkletProcessor {
+  constructor() {
+    super();
+    this.buffer = new Float32Array(0);
+    this.maxBuffered = 48000 / 5; // 200ms max
+    this.port.onmessage = (e) => {
+      const incoming = new Float32Array(e.data);
+      // Append
+      const newBuf = new Float32Array(this.buffer.length + incoming.length);
+      newBuf.set(this.buffer);
+      newBuf.set(incoming, this.buffer.length);
+      this.buffer = newBuf;
+
+      // Cap buffer to prevent drift
+      if (this.buffer.length > this.maxBuffered) {
+        this.buffer = this.buffer.slice(this.buffer.length - this.maxBuffered);
+      }
+    };
+  }
+
+  process(inputs, outputs, parameters) {
+    const output = outputs[0];
+    if (!output || !output[0]) return true;
+
+    const out = output[0]; // 128 samples typically
+
+    if (this.buffer.length >= out.length) {
+      out.set(this.buffer.subarray(0, out.length));
+      this.buffer = this.buffer.slice(out.length);
+    } else if (this.buffer.length > 0) {
+      out.set(this.buffer);
+      for (let i = this.buffer.length; i < out.length; i++) out[i] = 0;
+      this.buffer = new Float32Array(0);
+    } else {
+      for (let i = 0; i < out.length; i++) out[i] = 0;
+    }
+
+    return true;
+  }
+}
+
+registerProcessor('playback-processor', PlaybackProcessor);