chore(web): add Vitest; add dayCapabilities helper with full test coverage

- 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)

mise.toml

@@ -44,6 +44,15 @@ run = "npm run dev"
 description = "Run all Go tests"
 run = "go test ./..."
 
+[tasks."test:web"]
+description = "Run frontend Vitest unit tests"
+dir = "web"
+run = "npm test"
+
+[tasks."test:all"]
+description = "Run Go tests and frontend Vitest tests"
+depends = ["test", "test:web"]
+
 [tasks.install]
 description = "Install frontend npm dependencies"
 dir = "web"