fix: markdownToHtml multiline bold — [\s\S] instead of dot

GLM outputs multi-line **bold** blocks in essays. The dot regex .+?
does not match newlines, so multiline bold rendered as raw **.

src/bot/message-sender.js

@@ -63,7 +63,7 @@ export function markdownToHtml(text) {
 
   // 4. Convert Markdown patterns → HTML tags
   text = text
-    .replace(/\*\*(.+?)\*\*/g, '<b>$1</b>')          // **bold**
+    .replace(/\*\*([\s\S]+?)\*\*/g, '<b>$1</b>')          // **bold** (multiline)
     .replace(/(?<!\*)\*(?!\*)(.+?)(?<!\*)\*(?!\*)/g, '<i>$1</i>')  // *italic* (not inside **)
     .replace(/~~(.+?)~~/g, '<s>$1</s>')              // ~~strike~~
     .replace(/\[(.+?)\]\((.+?)\)/g, '<a href="$2">$1</a>')  // [link](url)