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/shop/collections/page.tsx

@@ -1,5 +1,13 @@
+import type { Metadata } from 'next';
 import Collections from '@/components/shopify/collections';
 
+export const metadata: Metadata = {
+  title: 'Collections',
+  description: 'Browse every collection in the store.',
+  // Same listing as /collections; point crawlers at the canonical route.
+  alternates: { canonical: '/collections' },
+};
+
 export default function Page() {
   return <Collections title="Our Collections" />;
 }