feat(ui): right-rail companion chat — streaming, tool chips, inline drafts

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/markdown.js

@@ -0,0 +1,16 @@
+// Shared markdown → sanitized HTML helper.
+// Used by the companion chat rail and any other component that needs
+// safe rendered markdown outside the full editor.
+
+import { marked } from './vendor/marked.esm.js';
+import DOMPurify from './vendor/purify.es.mjs';
+
+marked.setOptions({ gfm: true, breaks: true });
+
+/**
+ * Parse `src` as Markdown and return a DOMPurify-sanitized HTML string.
+ * Safe to assign to element.innerHTML.
+ */
+export function renderMarkdown(src) {
+  return DOMPurify.sanitize(marked.parse(src || ''));
+}