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/vite.config.ts

@@ -1,5 +1,5 @@
 import { sveltekit } from '@sveltejs/kit/vite';
-import { defineConfig } from 'vite';
+import { defineConfig } from 'vitest/config';
 import { VitePWA } from 'vite-plugin-pwa';
 
 export default defineConfig({
@@ -45,5 +45,10 @@ export default defineConfig({
 			'/api': 'http://localhost:8080',
 			'/healthz': 'http://localhost:8080'
 		}
+	},
+	test: {
+		environment: 'jsdom',
+		globals: true,
+		include: ['src/**/*.test.ts']
 	}
 });