seo: add canonical URLs + alternates to per-page generateMetadata

[bntvllnt]

Problem

No canonical tags emitted. Risk of duplicate-content treatment when component pages are reachable via search params, trailing slashes, or future i18n routes. Also no `alternates` map.

Goal

Every page emits a canonical URL.

Acceptance criteria

• `lib/og.ts` (or new `lib/seo.ts`) exposes `canonical(path)` helper using `metadataBase`.
• Each `generateMetadata` adds `alternates: { canonical: canonical(path) }`:
  • `app/page.tsx` → `/`
  • `app/components/page.tsx` → `/components`
  • `app/components/[slug]/page.tsx` → `/components/{slug}`
  • `app/docs/page.tsx` → `/docs`
  • `app/philosophy/page.tsx` → `/philosophy`
• Verify via `curl -s https://ui.vllnt.ai/components/button | grep canonical` after deploy.