feat: rich text editor, image crop component, empty DB resilience

- RichTextarea with toolbar (Bold, Italic, List, Heading) + Ctrl+B/I
  hotkeys (layout-independent), active state highlighting, preview mode
- Shared ImageCropField component (replaces duplicate in news/classes)
  with drag-to-reposition, Ctrl+scroll zoom, compact layout
- SectionEditor defaultData prop — all admin pages handle empty DB
- Team: section title editable, toast notifications, unsaved data warning
  on navigation (back button, sidebar links, browser close)
- Carousel: continuous card wrapping during drag, edge fade for small teams
- Markup renderer: **bold**, *italic*, ## headings, 🤍 bullet points
- Empty DB guards on all public site sections
- Fix: upload error handling, contact phone field, "team" section key

src/app/admin/master-classes/page.tsx

@@ -496,6 +496,7 @@ export default function MasterClassesEditorPage() {
     <SectionEditor<MasterClassesData>
       sectionKey="masterClasses"
       title="Мастер-классы"
+      defaultData={{ items: [] }}
     >
       {(data, update) => {
         // Sort: active first, archived at bottom