Extract line number gutter width calculation and improve text selection

packages/app/src/components/code-insets.ts

@@ -1,3 +1,12 @@
+/**
+ * Compute the pixel width for a line-number gutter based on the highest
+ * line number that will be displayed. Minimum width accommodates 2 digits.
+ */
+export function lineNumberGutterWidth(maxLineNumber: number): number {
+  const digits = Math.max(2, String(maxLineNumber).length);
+  return digits * 8 + 12;
+}
+
 export function getCodeInsets(theme: any) {
   const padding =
     typeof theme.spacing?.[3] === "number"