test(04-01): add failing tests for locale-aware formatCurrency

- Test English default locale uses comma grouping (1,234.56)
- Test explicit 'en' locale formatting
- Test 'de' locale uses period grouping and comma decimal (1.234,56)
- Test USD with dollar sign
- Test zero and negative amounts
- Test empty string locale fallback (no RangeError)
- Test default arg does NOT produce German formatting

frontend/src/lib/format.test.ts

@@ -0,0 +1,46 @@
+import { describe, it, expect } from 'vitest'
+import { formatCurrency } from './format'
+
+describe('formatCurrency', () => {
+  it('formats with English locale by default', () => {
+    const result = formatCurrency(1234.56, 'EUR')
+    expect(result).toContain('1,234.56')
+  })
+
+  it('formats with explicit English locale using comma grouping and period decimal', () => {
+    const result = formatCurrency(1234.56, 'EUR', 'en')
+    expect(result).toContain('1,234.56')
+  })
+
+  it('formats with German locale using period grouping and comma decimal', () => {
+    const result = formatCurrency(1234.56, 'EUR', 'de')
+    expect(result).toContain('1.234,56')
+  })
+
+  it('formats USD with English locale including dollar sign', () => {
+    const result = formatCurrency(1234.56, 'USD', 'en')
+    expect(result).toContain('$')
+    expect(result).toContain('1,234.56')
+  })
+
+  it('formats zero correctly', () => {
+    const result = formatCurrency(0, 'EUR', 'en')
+    expect(result).toContain('0.00')
+  })
+
+  it('formats negative amounts correctly', () => {
+    const result = formatCurrency(-500, 'EUR', 'en')
+    expect(result).toContain('-')
+    expect(result).toContain('500.00')
+  })
+
+  it('falls back gracefully when locale is empty string (does not throw)', () => {
+    expect(() => formatCurrency(1234.56, 'EUR', '')).not.toThrow()
+  })
+
+  it('does NOT produce German formatting when called without locale arg', () => {
+    const result = formatCurrency(1234.56, 'EUR')
+    // English default should have comma grouping, not period grouping
+    expect(result).not.toContain('1.234,56')
+  })
+})