feat: placeholder images, Nostr inbox, order lookup, SEO, bigger logo

- 5 SVG placeholder product images (minimal dark style with watermark initials)
- Seed data updated to reference .svg placeholders
- Nostr DM inbox in admin (Messages tab) with shop npub display
- GET /api/admin/nostr-info endpoint for shop pubkey
- My Orders page: customers look up orders by NIP-07 Nostr identity
- GET /api/orders/by-pubkey/:pubkey endpoint with hex validation
- SeoMeta component for OG/Twitter meta tags
- SEO meta on HomeView and ProductView
- Base OG meta tags in index.html
- "My Orders" link in shop header nav
- Splash logo doubled in size on desktop (680px max)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/admin/AdminShell.vue

@@ -23,6 +23,7 @@ async function handleLogout() {
       <nav class="sidebar-nav">
         <router-link :to="{ name: 'admin-orders' }" class="nav-item">Orders</router-link>
         <router-link :to="{ name: 'admin-products' }" class="nav-item">Products</router-link>
+        <router-link :to="{ name: 'admin-messages' }" class="nav-item">Messages</router-link>
       </nav>
 
       <div class="sidebar-footer">