Add a JS unit test runner (vitest) for client-side logic #1602

The frontend (EssentialCSharp.Web/src) has no JS unit test setup; package.json only has vite and the Vue plugin, so pure helpers and composables are untested.

Proposal: add vitest (plus @vue/test-utils if component tests are wanted), a test script, and run it in CI.

Surfaced while reviewing #1267 (reading time tracker).