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/layout.tsx

@@ -1,4 +1,5 @@
 import React from 'react';
+import type { Metadata } from 'next';
 import './globals.css';
 import { Geist, Geist_Mono } from 'next/font/google';
 import Header from '@/components/shopify/header';
@@ -16,6 +17,29 @@ const geistMono = Geist_Mono({
   variable: '--font-geist-mono',
 });
 
+// Routes only set their own `title`; the template appends the store name, and
+// everything else here is inherited unless a page overrides it.
+export const metadata: Metadata = {
+  metadataBase: new URL(site.url),
+  title: {
+    default: site.storeName,
+    template: `%s — ${site.storeName}`,
+  },
+  description: site.description,
+  openGraph: {
+    type: 'website',
+    siteName: site.storeName,
+    title: site.storeName,
+    description: site.description,
+    url: '/',
+  },
+  twitter: {
+    card: 'summary_large_image',
+    title: site.storeName,
+    description: site.description,
+  },
+};
+
 export default function RootLayout({
   children,
 }: {