Restructure into asr/tts packages and add TTS tab skeleton

Split the single ASR gateway into app/main.py (entrypoint) + app/core
(shared env helpers) + app/asr (all existing ASR logic, unchanged
behavior) + app/tts (placeholder router/config, no engine yet), so a
real TTS backend can be dropped in later without reshuffling ASR code.
API moved under /asr/* and /tts/* prefixes; UI gained a top-level
ASR/TTS tab switcher built on a reusable nested .tab-group mechanism,
with JS split into common.js/asr.js/tts.js.

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

app/ui/assets/js/pcm-processor.js

@@ -0,0 +1,29 @@
+// AudioWorklet processor — accumulates 4096 samples (~256ms at 16kHz) before sending
+class PCMProcessor extends AudioWorkletProcessor {
+  constructor() {
+    super();
+    this._chunks = [];
+    this._total = 0;
+    this._target = 4096;
+  }
+
+  process(inputs) {
+    const ch = inputs[0] && inputs[0][0];
+    if (ch && ch.length > 0) {
+      this._chunks.push(ch.slice());
+      this._total += ch.length;
+
+      if (this._total >= this._target) {
+        const out = new Float32Array(this._total);
+        let offset = 0;
+        for (const c of this._chunks) { out.set(c, offset); offset += c.length; }
+        this._chunks = [];
+        this._total = 0;
+        this.port.postMessage(out.buffer, [out.buffer]);
+      }
+    }
+    return true;
+  }
+}
+
+registerProcessor('pcm-processor', PCMProcessor);