Add Testing Framework and Unit Tests

[sondt99]

The project currently has no tests or testing framework configured. This creates risk for regressions when modifying the markdown pipeline, utilities, or components.

Proposed Solution

• Add Vitest or Jest as the testing framework
• Add @testing-library/react for component testing
• Create unit tests for:
  • src/lib/markdown.ts - post parsing, statistics calculation, tag normalization
  • src/lib/content-assets.ts - URL resolution
  • Component rendering (TableOfContents, TagList, Timeline)

Acceptance Criteria

• Testing framework configured
• Tests for markdown utilities
• Tests for components
• npm test script works