feat: add news section with admin editor and public display

- NewsItem type with title, text, date, optional image and link
- Admin page at /admin/news with image upload and auto-date
- Public section between Pricing and FAQ, hidden when empty
- Nav link auto-hides when no news items exist

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/data/content.ts

@@ -442,6 +442,10 @@ export const siteContent: SiteContent = {
       },
     ],
   },
+  news: {
+    title: "Новости",
+    items: [],
+  },
   contact: {
     title: "Контакты",
     addresses: [