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>

tests/ui/math.test.ts

@@ -2,6 +2,7 @@ import { describe, expect, it } from 'vitest';
 import {
   clamp,
   centreY,
+  formatBytes,
   formatNumber,
   NUMBER_DECIMALS,
   roundTo,
@@ -122,3 +123,16 @@ describe('zoomFactor', () => {
     expect(1 * zoomFactor(-120) * zoomFactor(120)).toBeCloseTo(1, 12);
   });
 });
+
+describe('formatBytes', () => {
+  it('names a small file in bytes, so a sprite never reads as "0 KB"', () => {
+    expect(formatBytes(400)).toBe('400 B');
+    expect(formatBytes(0)).toBe('0 B');
+  });
+
+  it('switches to KB and MB at the round thresholds', () => {
+    expect(formatBytes(1024)).toBe('1 KB');
+    expect(formatBytes(1024 * 1024)).toBe('1.0 MB');
+    expect(formatBytes(1536 * 1024)).toBe('1.5 MB');
+  });
+});