feat: add JSON-LD structured data for product and site pages

JSON-LD was absent entirely (sitemap generation is separate backend
work, out of scope here). Added Product schema (name/description/
image/offers with price+availability) on item pages via setItemMeta(),
and a site-wide Organization schema via resetToDefaults(), both
injected as a single #seo-json-ld <script type=application/ld+json>
tag that gets replaced on navigation.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/services/seo.service.ts

@@ -86,6 +86,22 @@ export class SeoService {
       // Standard meta
       { name: 'description', content: description },
     ]);
+
+    this.setJsonLd({
+      '@context': 'https://schema.org',
+      '@type': 'Product',
+      name: item.name,
+      description,
+      image: imageUrl,
+      url: itemUrl,
+      offers: {
+        '@type': 'Offer',
+        price: price.toFixed(2),
+        priceCurrency: item.currency || 'RUB',
+        availability: (item.quantity ?? 0) > 0 ? 'https://schema.org/InStock' : 'https://schema.org/OutOfStock',
+        url: itemUrl,
+      },
+    });
   }
 
   /**
@@ -139,6 +155,30 @@ export class SeoService {
     // Remove product-specific tags
     this.meta.removeTag("property='product:price:amount'");
     this.meta.removeTag("property='product:price:currency'");
+
+    this.setJsonLd({
+      '@context': 'https://schema.org',
+      '@type': 'Organization',
+      name: this.siteName,
+      url: this.siteUrl,
+      ...(defaultImage ? { logo: defaultImage } : {}),
+    });
+  }
+
+  /** Replace (or remove, when data is null) the page's JSON-LD structured-data script tag. */
+  private setJsonLd(data: Record<string, unknown> | null): void {
+    const existing = this.doc.getElementById('seo-json-ld');
+    existing?.remove();
+
+    if (!data) {
+      return;
+    }
+
+    const script = this.doc.createElement('script');
+    script.id = 'seo-json-ld';
+    script.type = 'application/ld+json';
+    script.text = JSON.stringify(data);
+    this.doc.head.appendChild(script);
   }
 
   private setOrUpdate(tags: Array<{ property?: string; name?: string; content: string }>): void {