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)

web/package.json

@@ -7,6 +7,8 @@
 		"dev": "vite dev",
 		"build": "vite build",
 		"preview": "vite preview",
+		"test": "vitest run",
+		"test:watch": "vitest",
 		"prepare": "svelte-kit sync || echo ''",
 		"check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json",
 		"check:watch": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json --watch"
@@ -15,10 +17,12 @@
 		"@sveltejs/adapter-auto": "^7.0.1",
 		"@sveltejs/kit": "^2.57.0",
 		"@sveltejs/vite-plugin-svelte": "^7.0.0",
+		"jsdom": "^29.1.1",
 		"svelte": "^5.55.2",
 		"svelte-check": "^4.4.6",
 		"typescript": "^6.0.2",
-		"vite": "^8.0.7"
+		"vite": "^8.0.7",
+		"vitest": "^4.1.5"
 	},
 	"dependencies": {
 		"@sveltejs/adapter-static": "^3.0.10",