Add unit and integration test coverage

[vayungodara]

Problem

Only 2 e2e test files exist and zero unit tests. This makes refactoring risky and slows development confidence.

What to change

• Set up Jest or Vitest for unit testing
• Add unit tests for critical business logic: streak calculations, pact validation, group membership rules
• Add integration tests for key API routes / server actions
• Target: at least 1 test file per lib/ module to start

Acceptance criteria

• Test runner configured and documented in README
• Unit tests for streak logic
• Unit tests for pact validation
• Tests run in CI pipeline
• npm test works locally

Label: enhancement | Priority: high

[vayungodara]

Closed — 104 unit tests via Vitest across 6 test files (gamification, streaks, streaks-advanced, pactTemplates, activity, reactions). CI pipeline running on every push.