fix(app): keep diff rows aligned for blank lines

packages/app/src/utils/diff-rendering.ts

@@ -0,0 +1,16 @@
+interface HighlightLikeToken {
+  text: string;
+}
+
+// Preserve row height when a gutter or diff cell is intentionally blank.
+export function formatDiffGutterText(lineNumber: number | null): string {
+  return lineNumber == null ? " " : String(lineNumber);
+}
+
+export function formatDiffContentText(content: string | null | undefined): string {
+  return content && content.length > 0 ? content : " ";
+}
+
+export function hasVisibleDiffTokens(tokens: HighlightLikeToken[] | null | undefined): boolean {
+  return Boolean(tokens?.some((token) => token.text.length > 0));
+}