fix: strip markdown for TTS + render rich markdown in chat UI

- _md_to_speech(): AST-based markdown stripping via markdown-it-py
- Truncate TTS input at 450 chars on sentence boundary (chatterbox overflow)
- chat.js: render assistant messages as markdown via marked.js + DOMPurify
- Typewriter accumulates raw text, renders markdown progressively

app/static/chat.js

@@ -21,9 +21,22 @@ let ws = null;
 let connectAttempts = 0;
 let lastSpeaker = null;       // 'user' | 'bt' | 'system' | null
 let currentBtBody = null;     // active streaming .msg__body element
+let currentBtRaw = '';        // raw markdown accumulator for streaming
 let queue = [];
 let draining = false;
 
+// Configure marked for safe markdown rendering
+if (typeof marked !== 'undefined') {
+  marked.setOptions({ breaks: true, gfm: true });
+}
+
+function renderMarkdown(raw) {
+  if (typeof marked === 'undefined') return raw;
+  const html = marked.parse(raw);
+  if (typeof DOMPurify !== 'undefined') return DOMPurify.sanitize(html);
+  return html;
+}
+
 // ---------- helpers ----------
 
 function speakerLabel(role) {
@@ -97,7 +110,9 @@ async function drain() {
   if (!currentBtBody) return;
   draining = true;
   while (queue.length) {
-    currentBtBody.textContent += queue.shift();
+    currentBtRaw += queue.shift();
+    // Re-render markdown on each char (marked.parse is fast enough)
+    currentBtBody.innerHTML = renderMarkdown(currentBtRaw);
     if (Math.random() < 0.04) scrollToBottom();
     await sleep(TYPEWRITER_MS);
   }
@@ -107,16 +122,19 @@ async function drain() {
 function sleep(ms) { return new Promise(r => setTimeout(r, ms)); }
 
 function finishBt() {
-  // wait for the queue to drain before adding caret
+  // wait for the queue to drain before final render
   const tick = () => {
     if (draining || queue.length) { setTimeout(tick, 30); return; }
     if (!currentBtBody) return;
+    // Final markdown render with complete text
+    currentBtBody.innerHTML = renderMarkdown(currentBtRaw);
     const caret = document.createElement('span');
     caret.className = 'caret';
     currentBtBody.appendChild(caret);
     scrollToBottom();
     setTimeout(() => caret.remove(), 900);
     currentBtBody = null;
+    currentBtRaw = '';
   };
   tick();
 }
@@ -188,6 +206,7 @@ function handleMessage(msg) {
     if (!currentBtBody) {
       removeThinking();
       currentBtBody = makeMsg('bt');
+      currentBtRaw = '';
     }
     if (msg.delta) enqueue(msg.delta);
     if (msg.done) finishBt();