fix(format): divide by 1e9 in formatLongNumber billions branch

The billions branch of formatLongNumber divided by 1000000 while
appending the 'b' suffix, so values >= 1e9 rendered about 1000x too
large (1000000000 became '1000.0b' instead of '1.0b'). Divide by
1000000000 to match the 'b' unit, consistent with the sibling
formatLongCurrency. Add regression tests for the billions branch.

src/lib/format.test.ts

@@ -27,6 +27,8 @@ test('formatNumber', () => {
 });
 
 test('formatLongNumber', () => {
+  expect(format.formatLongNumber(1000000000)).toBe('1.0b');
+  expect(format.formatLongNumber(2500000000)).toBe('2.5b');
   expect(format.formatLongNumber(1200000)).toBe('1.2m');
   expect(format.formatLongNumber(575000)).toBe('575k');
   expect(format.formatLongNumber(10500)).toBe('10.5k');

src/lib/format.ts

@@ -51,7 +51,7 @@ export function formatLongNumber(value: number) {
   const n = Number(value);
 
   if (n >= 1000000000) {
-    return `${(n / 1000000).toFixed(1)}b`;
+    return `${(n / 1000000000).toFixed(1)}b`;
   }
   if (n >= 1000000) {
     return `${(n / 1000000).toFixed(1)}m`;