Add vitest test infrastructure with initial importer tests

[ventilla]

Summary

• Sets up vitest as the test runner with jsdom environment
• Adds a lightweight mock of the obsidian module for unit testing
• Adds initial tests for three areas:
  • Google Keep importer (20 tests): tag sanitization, text/list conversion, labels, colors, attachments
  • Apple Journal importer (20 tests): golden file comparison against existing fixtures, date extraction, front matter tokens
  • Shared utilities (15 tests): sanitizeFileName, parseHTML, serializeFrontMatter, truncateText

Motivation

The repo has excellent test fixture files in tests/ but no automated test runner. This PR adds the infrastructure so that:

• Contributors can run npm test to validate changes
• CI can be added later to catch regressions
• The existing fixtures (Keep JSON, Apple Journal HTML/MD pairs) are exercised automatically

Test plan

• npx vitest run — all 55 tests pass
• npx eslint src/**/*.test.ts — no lint violations
• No changes to existing source files — purely additive
• Reviewer: verify tests match expected importer behavior

Notes

• Tests are colocated with source (src/formats/*.test.ts) following vitest conventions
• The obsidian mock (src/__mocks__/obsidian.ts) stubs UI classes and provides faithful implementations of utility functions (stringifyYaml, htmlToMarkdown, moment, normalizePath)
• Apple Journal tests document a minor gap: the current importer collects a "Health" token via .gridItemOverlayFooter that the fixture .md files omit