fix(app): keep diff gutter row aligned when line number is blank

The recently added numberOfLines={1} on the gutter <Text> wraps the
content in display:-webkit-box; overflow:hidden on web, where a plain
ASCII space collapses to zero height. That shifted every line number up
by one row, leaving "1" next to the hunk marker. Use a non-breaking
space so the cell keeps its line height.

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

@@ -3,8 +3,10 @@ interface HighlightLikeToken {
 }
 
 // Preserve row height when a gutter or diff cell is intentionally blank.
+// Non-breaking space because the gutter <Text> uses numberOfLines={1}, which
+// collapses a plain ASCII space to zero height on web.
 export function formatDiffGutterText(lineNumber: number | null): string {
-  return lineNumber == null ? " " : String(lineNumber);
+  return lineNumber == null ? "\u00A0" : String(lineNumber);
 }
 
 export function formatDiffContentText(content: string | null | undefined): string {