Store picker in topbar, redesign cabinet, rename to WBfeed.ru

- index.html: store dropdown in topbar (switch without leaving page)
- cabinet.html: store-cards with Активировать/Проверить, edit hidden in <details>
- Removed 'Используется' button, active shown as green card + label
- next=index param to return to main page after store switch
- Brand name changed to WBfeed.ru across all templates

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

templates/admin.html

@@ -12,7 +12,7 @@
     <div class="topbar__inner">
         <a href="{{ url_for('index') }}" class="topbar__brand">
             <img src="{{ url_for('static', filename='wb3.png') }}" class="topbar__logo-img" alt="WB">
-            <span class="topbar__name">Feedback</span>
+            <span class="topbar__name">WBfeed.ru</span>
         </a>
         <div class="topbar__nav">
             <a href="{{ url_for('index') }}" class="topbar__link">Отзывы</a>