Add cacheable Danish website translation

[kasnder]

Summary

• Add Danish translations across public website pages, report states, metadata, labels, dates, and browser filter messages.
• Serve Danish pages at stable /da/... URLs so English and Danish HTML can retain separate Cloudflare cache entries. Keep analyser and API endpoints unchanged.
• Add a language toggle, locale-preserving links and redirects, and purge changed Cloudflare pages in both languages after an analysis.
• Document the additional Danish search and request paths that need Cloudflare WAF challenges before deployment.

Verification

• node --test test/i18n.test.js test/cloudflarePurge.test.js — 11 passed.
• JavaScript syntax checks and git diff --check passed.
• Full Pug/request tests require repository dependencies, which are absent in this worktree. CI runs the complete suite.

Deployment note

Update the two Cloudflare WAF challenge expressions as shown in README before deploying so /da/search and /da/request/... receive the same protection as the English routes. Purge existing HTML after deployment so cached English pages pick up the new toggle.