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/search/page.tsx

@@ -2,7 +2,8 @@ import { Suspense } from 'react';
 import SearchResults from '@/components/shopify/search-results';
 
 export const metadata = {
-  title: 'Search — Shop',
+  title: 'Search',
+  description: 'Search the catalogue by product name, description or type.',
 };
 
 export default function Page() {