Add unit and integration test coverage

[vayungodara]

Problem

Only 2 e2e test files exist and zero unit tests. No test coverage for business logic like streaks, XP calculations, pact management.

What to change

• Set up Jest + React Testing Library (or Vitest if preferred)
• Add unit tests for critical lib/ utilities: streaks.js, xp.js, badges.js, dateUtils.js
• Add component tests for key UI: PactCard, TaskCard, FocusTimer, MonthlyCalendar
• Add integration tests for auth flows and pact creation
• Target 60%+ coverage on lib/ as a starting milestone

Impact

Catches regressions early, enables confident refactoring.

Effort

HIGH — ongoing, but initial setup ~4-6 hours

[vayungodara]

Closing as duplicate — kept the more detailed version.

[vayungodara]

Closing as duplicate of #14