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

config/site.ts

@@ -3,6 +3,15 @@ import type { NavLink } from '@/components/shopify/header';
 /** Chrome shared by every route — rendered once in the root layout. */
 export const site = {
   storeName: 'Shop',
+  description:
+    'An agent-friendly Shopify storefront built with Next.js and Hydrogen.',
+  // Absolute origin behind `metadataBase`, so Open Graph images resolve to full
+  // URLs. Vercel injects VERCEL_PROJECT_PRODUCTION_URL on deployed builds.
+  url:
+    process.env.NEXT_PUBLIC_SITE_URL ??
+    (process.env.VERCEL_PROJECT_PRODUCTION_URL
+      ? `https://${process.env.VERCEL_PROJECT_PRODUCTION_URL}`
+      : 'http://localhost:3000'),
   logoUrl: '',
   copyright: '© 2026 Shop. All rights reserved.',
   navLinks: [