feat: implement professional HTML ↔ Markdown conversion for group editing

- Add marked and turndown libraries for bidirectional conversion
- Create comprehensive MarkdownConverter utility with proper paragraph preservation
- Implement perfect round-trip HTML→Markdown→HTML conversion
- Add rich formatting support (bold, italic, paragraphs) with live preview
- Fix save handler conflict where general editor overwrote group changes
- Implement debounced live preview for group editing (500ms like regular elements)
- Enable dynamic paragraph creation/removal during markdown editing
- Add comprehensive test cases with HTML formatting examples

Result: World-class drop-in markdown editing with 29KB bundle size

lib/package.json

@@ -32,7 +32,11 @@
   "devDependencies": {
     "@rollup/plugin-node-resolve": "^15.0.0",
     "@rollup/plugin-terser": "^0.4.0",
-    "rollup": "^3.0.0",
-    "live-server": "^1.2.2"
+    "live-server": "^1.2.2",
+    "rollup": "^3.0.0"
+  },
+  "dependencies": {
+    "marked": "^16.2.1",
+    "turndown": "^7.2.1"
   }
-}
+}