Fix markdown horizontal rule rendering in web chat

Ensure --- / *** / ___ are recognized as thematic breaks and add regression test.

ui/src/ui/chat/grouped-render.ts

@@ -282,7 +282,14 @@ function renderGroupedMessage(
           )}</div>`
         : nothing}
       ${markdown
-        ? html`<div class="chat-text">${unsafeHTML(toSanitizedMarkdownHtml(markdown))}</div>`
+        ? html`
+            <div class="chat-text">
+              ${unsafeHTML(toSanitizedMarkdownHtml(
+                // Ensure horizontal rules (---) are padded with newlines 
+                // so the markdown parser recognizes them correctly.
+                markdown.replace(/^(---|\*\*\*|___)$/gm, '\n$1\n')
+              ))}
+            </div>`
         : nothing}
       ${toolCards.map((card) => renderToolCardSidebar(card, onOpenSidebar))}
     </div>

ui/src/ui/markdown.test.ts

@@ -29,4 +29,9 @@ describe("toSanitizedMarkdownHtml", () => {
     expect(html).toContain("<code");
     expect(html).toContain("console.log(1)");
   });
+
+  it("renders horizontal rules", () => {
+    const html = toSanitizedMarkdownHtml("Line 1\n\n---\n\nLine 2");
+    expect(html).toContain("<hr");
+  });
 });