Share the one HTTP envelope and the one byte formatter

Both stores carried their own ~28-line request<T>, and formatBytes existed
twice – the second copy had lost the "B" branch, so a small sprite would have
read as 0 KB had its caller ever passed one under a kilobyte.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/ui/math.ts

@@ -42,3 +42,9 @@ export function snapToStep(value: number, stepSize: number): number {
 export function zoomFactor(deltaY: number): number {
   return Math.exp(-deltaY * 0.0015);
 }
+
+export function formatBytes(bytes: number): string {
+  if (bytes >= 1024 * 1024) return `${(bytes / 1024 / 1024).toFixed(1)} MB`;
+  // A small sprite is a few hundred bytes, and "0 KB" reads like a broken file.
+  return bytes >= 1024 ? `${Math.round(bytes / 1024)} KB` : `${bytes} B`;
+}