- Install vitest + jsdom
- Add test/test:watch scripts to package.json
- Add test:web and test:all tasks to mise.toml
- Add dayCapabilities() to utils.ts — single source of truth for
what actions are permitted per day (future/today/past, open/closed)
- Add DayCapabilities interface to utils.ts
- 11 unit tests: dayCapabilities (5 cases), weekDayKeys (3 cases),
isWorkday (3 cases)