feat(renderer): PreviewPane with empty state and MarkdownRenderer

src/renderer/components/preview/PreviewPane.tsx

@@ -0,0 +1,20 @@
+import { MarkdownRenderer } from './MarkdownRenderer';
+import { usePreviewStore } from '@/stores/preview-store';
+
+export function PreviewPane() {
+  const { source } = usePreviewStore();
+
+  if (!source) {
+    return (
+      <div className="flex h-full items-center justify-center text-muted-foreground">
+        <p>Nothing to preview. Start typing in the editor.</p>
+      </div>
+    );
+  }
+
+  return (
+    <div className="h-full overflow-auto bg-card/10">
+      <MarkdownRenderer source={source} />
+    </div>
+  );
+}