Add tests and CI integration

Vitest is already installed (^4.0.18) but has no configuration, no test scripts, and no test files. This is the umbrella task, split into 4 sub-tasks that should be done in order.

Sub-tasks

1. Set up testing environment and CI -- vitest config, scripts, CI pipeline step
2. Unit tests for utility functions -- escape.js, url-utils.js, scroll-to.js
3. Unit tests for endpoints and renderer -- sitemap, feed, page-renderer
4. Component tests -- Svelte component rendering with @testing-library/svelte

Execution order

Task 1 (environment setup) must be done first. Tasks 2 and 3 can be done in parallel after that. Task 4 (component tests) depends on task 1 and may require additional dependencies (@testing-library/svelte).

E2E tests (future consideration)

Playwright for full browser testing. Lower priority since this is a static site, but useful for interactive components.

• Homepage loads and renders
• Navigation works (links, breadcrumbs)
• Todo listing page shows all todos
• Individual todo pages render content
• Interactive components: Clock, Timer, Snake game, Pinball
• Accordion expand/collapse
• Smooth scroll behavior