Backporting foundational work from Procens

[pixelcmtd]

The 1.0 we'll end up shipping will probably look quite different from Procens (#8), but the feature set will none-the-less be comparable. @CMJanina might not have inspected everything, but at least gave me this list to start with:

• Versioned template route. internal/templates/[id]/[version]/ + selector dropdown, backed by existing getTemplateVersion. Skip the getOwnCheck prefill → Add versioned template routes and revision selector #42
• LinkCard, CheckCard, Button. Pure UI; adapt CheckCard props to zod types (revision, TRData)
• index.astro landing. Template list of CheckCards with default ratings, no per-user prefill
• Timestamps on check pages. Show created_at on internal/checks/[id] (Procens prettyDate equivalent) → show created_at on check pages #40
• Old KCCs into content collection. Extract revisions from old_kccs.astro as template files, validate with templates tests, not the page itself