test(docs): add Vitest + astro check infrastructure

Frontend review flagged zero CI safety net for the visualizer. This
addresses the testable surface; Playwright e2e deferred (browser
download adds ~100MB to the lockfile and is better as a separate
PR).

- npm scripts: check (astro check), test (vitest run), test:watch,
  test:ui.
- vitest.config.ts: jsdom env, src/**/*.test.ts include glob.
- src/utils/share-url.test.ts: 15 cases covering encode/decode
  round-trip, hash reading, permalink build, Mermaid generation,
  Go-builder synthesis, and a no-localStorage-dep contract pin.
- devDeps: vitest, @vitest/ui, @astrojs/check, jsdom, typescript.

`npm run check` reports 0 errors, 0 warnings, 2 hints (deprecated
unescape — see followup task).
`npm test` reports 15/15 passing in <500ms.

Source: 4-expert website review — Frontend dimension top-3 ROI.

Author: felixgeelhaar