Add unit tests for lib/format-date.ts

[NiveditJain]

lib/format-date.ts exports a single formatDate(date: Date): string helper that wraps Intl.DateTimeFormat to produce strings like "Jan 15, 2024, 3:45 PM". It currently has no unit test even though sibling utilities like format-duration.ts are well-covered.

1. Create __tests__/lib/format-date.test.ts — copy the structure of __tests__/lib/format-duration.test.ts as a template.
2. Import: import { formatDate } from "@/lib/format-date";
3. Add cases for: a fixed UTC date (e.g. new Date(Date.UTC(2024, 0, 15, 15, 45))), the start of a month, midnight, single-digit day, and end of year. Assert against the exact expected string.
4. Run bun run test:run and confirm all cases pass.

Out of scope: changing format-date.ts itself, or adding locale handling. Just lock in current behavior.