Add page metadata across every route

The root layout exported no metadata, so routes without their own export
rendered no title at all, and no route had a description. Add a root
metadata block with a title template, Open Graph and Twitter defaults,
and fill in the routes that were missing tags.

- config/site.ts: description and env-driven absolute url for metadataBase
- home, /collections and /shop/collections: titles and descriptions
- products/[handle] and collections/[handle]: generateMetadata off the
  existing catalog fetchers, with canonical and OG image
- existing titles: drop the hand-written " — Shop" suffix now that the
  root template appends it

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011x5jT4fJJCnqPx3umEvXRk

app/account/activate/[id]/[token]/page.tsx

@@ -1,6 +1,6 @@
 import AccountForm from '@/components/shopify/account-form';
 
-export const metadata = { title: 'Activate your account — Shop' };
+export const metadata = { title: 'Activate your account' };
 
 // Shopify's emailed activation link is /account/activate/{id}/{token}.
 export default async function Page({