test: add cda-gui browser testing

[krowvin]

Summary

• add Storybook as a visible, isolated UI development surface for cda-gui
• run Storybook stories as browser tests with Vitest and Playwright Chromium
• add a landing-page story that verifies the introduction and primary navigation links render
• run the browser suite in the existing CDA Web GUI workflow and document local usage

This gives contributors a repeatable way to develop and verify GUI components without requiring a live CDA API.

Related Issue

N/A

Validation

• npm run test-storybook -- --run - 1 test passed in Chromium
• npm run build-storybook
• npm run build
• npm run lint
• npm run format:check
• npm audit --audit-level=critical - passed; npm reports 10 non-critical advisories

Checklist

• AI tools used