feat(format): add formatRelativeTime (just now, Xm/Xh/Xd ago, MM/DD/YYYY)

src/lib/__tests__/format-date.test.ts

@@ -0,0 +1,36 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+import { formatRelativeTime } from "../format-date";
+
+describe("formatRelativeTime", () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date("2026-06-17T12:00:00Z"));
+  });
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it("returns 'just now' for < 45s", () => {
+    expect(formatRelativeTime("2026-06-17T11:59:30Z")).toBe("just now");
+  });
+
+  it("returns '5m ago' for ~5 minutes", () => {
+    expect(formatRelativeTime("2026-06-17T11:55:00Z")).toBe("5m ago");
+  });
+
+  it("returns '2h ago' for ~2 hours", () => {
+    expect(formatRelativeTime("2026-06-17T10:00:00Z")).toBe("2h ago");
+  });
+
+  it("returns 'yesterday' for 1 day", () => {
+    expect(formatRelativeTime("2026-06-16T12:00:00Z")).toBe("yesterday");
+  });
+
+  it("returns '3d ago' for 3 days", () => {
+    expect(formatRelativeTime("2026-06-14T12:00:00Z")).toBe("3d ago");
+  });
+
+  it("returns MM/DD/YYYY for > 7 days", () => {
+    expect(formatRelativeTime("2026-06-01T12:00:00Z")).toBe("06/01/2026");
+  });
+});