fix(frontend): support nested preview links

[dongmucat]

What

• route markdown links inside previewed package files through the same package-relative resolver used by the overview
• keep resolving nested links against the currently previewed file path
• add unit and Playwright coverage for the second-level preview flow

Why

• follow up on issue [Feature] 概览能正常显示相对链接的内容 #500 comment: [Feature] 概览能正常显示相对链接的内容 #500 (comment)
• first-level links were fixed, but links inside the previewed markdown dialog still fell back to native browser navigation instead of opening the next package file

How

• extract a shared handlePackageMarkdownLinkClick(...) path in SkillDetailPage
• use documentationPath for overview links and previewNode?.path for links rendered inside FilePreviewDialog
• pass onLinkClick through FilePreviewDialog into MarkdownRenderer
• extend the existing relative-link page test and real-API Playwright test to cover README.md -> docs/usage.md -> docs/nested.md

Testing

• make typecheck-web
• make lint-web
• make test-backend-app
• cd web && npx -y node@20.19.0 ./node_modules/vitest/vitest.mjs run
• cd web && env -u http_proxy -u https_proxy -u HTTP_PROXY -u HTTPS_PROXY -u all_proxy -u ALL_PROXY pnpm exec playwright test e2e/skill-detail-relative-links.spec.ts

Validation Notes

• make test-frontend still fails on the local default Node v20.18.0 because Vitest/jsdom hits the known html-encoding-sniffer -> @exodus/bytes ESM mismatch; the full suite passes under node@20.19.0
• make staging currently fails in the web container startup path with /docker-entrypoint.d/20-envsubst-on-templates.sh: can't open /etc/nginx/templates/default.conf.template: Operation not permitted; this happened after backend image build and frontend static build succeeded

Impact

• nested relative links in previewed markdown files now open the next package file instead of leaving the in-app preview flow
• external links, absolute links, and same-document anchors keep their previous behavior