Add K/M/B scale toggles for financial displays

lib/format.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'bun:test';
-import { formatScaledNumber } from './format';
+import { formatCurrencyByScale, formatScaledNumber } from './format';
 
 describe('formatScaledNumber', () => {
   it('keeps values below one thousand unscaled', () => {
@@ -27,3 +27,21 @@ describe('formatScaledNumber', () => {
     expect(formatScaledNumber(999_950)).toBe('1M');
   });
 });
+
+describe('formatCurrencyByScale', () => {
+  it('formats values in thousands', () => {
+    expect(formatCurrencyByScale(12_345, 'thousands')).toBe('$12.3K');
+  });
+
+  it('formats values in millions', () => {
+    expect(formatCurrencyByScale(12_345_678, 'millions')).toBe('$12.3M');
+  });
+
+  it('formats values in billions', () => {
+    expect(formatCurrencyByScale(12_345_678_901, 'billions')).toBe('$12.3B');
+  });
+
+  it('keeps sign for negative values', () => {
+    expect(formatCurrencyByScale(-2_500_000, 'millions')).toBe('-$2.5M');
+  });
+});