Add automated test infrastructure for utility functions

[kmch4n]

Summary

The project relies entirely on manual verification. Adding a minimal test suite for pure utility functions would catch regressions with low maintenance cost.

Scope

• Unit tests for pure functions: dateUtils, geoUtils, calendarUtils, parseTechniques
• Repository tests using an in-memory SQLite database (optional, higher effort)

Suggested stack

Vitest or Jest, compatible with the existing Expo + TypeScript setup.

Notes

• Start small with utility functions — they have no native dependencies and are easy to test.
• Repository tests require more setup (in-memory DB, migrations) but would cover critical data paths.

Split from #36.