feat(frontend): add types + format helpers with tdd

frontend/src/tests/format.test.ts

@@ -0,0 +1,61 @@
+import { describe, expect, it } from 'vitest';
+import { fmtMs, fmtRelative, fmtJpDate, fmtJpTime } from '@lib/format';
+
+describe('fmtMs', () => {
+  it('formats zero as 0:00', () => {
+    expect(fmtMs(0)).toBe('0:00');
+  });
+  it('pads seconds to two digits', () => {
+    expect(fmtMs(7)).toBe('0:07');
+  });
+  it('formats 215 as 3:35', () => {
+    expect(fmtMs(215)).toBe('3:35');
+  });
+  it('handles minutes >= 10 correctly', () => {
+    expect(fmtMs(605)).toBe('10:05');
+  });
+  it('clamps negative input to 0:00', () => {
+    expect(fmtMs(-3)).toBe('0:00');
+  });
+  it('floors fractional seconds', () => {
+    expect(fmtMs(7.9)).toBe('0:07');
+  });
+});
+
+describe('fmtRelative', () => {
+  const now = new Date('2026-04-30T12:00:00Z');
+  it('formats <60s as Xs', () => {
+    expect(fmtRelative(new Date('2026-04-30T11:59:30Z'), now)).toBe('30s');
+  });
+  it('formats <1h as Xm', () => {
+    expect(fmtRelative(new Date('2026-04-30T11:55:00Z'), now)).toBe('5m');
+  });
+  it('formats <24h as Xh', () => {
+    expect(fmtRelative(new Date('2026-04-30T09:00:00Z'), now)).toBe('3h');
+  });
+  it('formats >=24h as Xd', () => {
+    expect(fmtRelative(new Date('2026-04-28T12:00:00Z'), now)).toBe('2d');
+  });
+  it('returns "now" for the current moment', () => {
+    expect(fmtRelative(new Date('2026-04-30T12:00:00Z'), now)).toBe('now');
+  });
+  it('returns "now" for a future timestamp (clock skew)', () => {
+    expect(fmtRelative(new Date('2026-04-30T12:00:05Z'), now)).toBe('now');
+  });
+});
+
+describe('fmtJpDate', () => {
+  it('formats with full kanji', () => {
+    // build the date in local time so the test isn't tz-sensitive
+    const d = new Date(2026, 3, 30); // month is 0-indexed → April
+    expect(fmtJpDate(d)).toBe('2026年4月30日');
+  });
+});
+
+describe('fmtJpTime', () => {
+  it('formats HH:MM:SS', () => {
+    const d = new Date();
+    d.setHours(7); d.setMinutes(3); d.setSeconds(9);
+    expect(fmtJpTime(d)).toBe('07:03:09');
+  });
+});