Add frontend test infrastructure

[dellch]

Summary

Add Vitest-based test infrastructure for the Vue 3 frontend to support the testing strategy outlined in CLAUDE.md.

Scope

• Install Vitest and testing dependencies (e.g., @vue/test-utils, jsdom)
• Create vitest.config.ts
• Add test scripts to package.json:
  • test:frontend — run tests once
  • test:frontend:watch — run in watch mode
• Write at least one minimal unit test to validate the setup
• Update Makefile with test-frontend target

Context

The frontend scaffold PR (#3) focused on build, lint, and type-check tooling. This issue tracks the separate effort to add the test layer mentioned in the testing strategy.

Related

• PR Scaffold Vue 3 frontend #3
• Review comment: Scaffold Vue 3 frontend #3 (comment)