Add diff viewer to git commit dialog (#374)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

src-web/lib/diffYaml.ts

@@ -0,0 +1,15 @@
+import type { SyncModel } from '@yaakapp-internal/git';
+import { stringify } from 'yaml';
+
+/**
+ * Convert a SyncModel to a clean YAML string for diffing.
+ * Removes noisy fields like updatedAt that change on every edit.
+ */
+export function modelToYaml(model: SyncModel | null): string {
+  if (!model) return '';
+
+  return stringify(model, {
+    indent: 2,
+    lineWidth: 0,
+  });
+}