fix: harden docs webapp links and default GitHub Pages host

[Gaubee]

Summary

• default DWEB site origin/base to GitHub Pages (https://{owner}.github.io/{repo}/) when env is absent
• remove SITE_ORIGIN variable override in CD workflow (keep deterministic GitHub Pages host)
• generate webapp and webapp-dev zip artifacts in docs publish flow for download page
• add Web ZIP links on download page
• open webapp links in new tab to avoid VitePress SPA router interception/404

Verification

• pnpm docs:build
• pnpm vitest run src/lib/__tests__/dweb-update.test.ts --reporter=dot