feat: add product interaction mode selector with three scenarios

- Add ItemToggleButton component for 3-way toggle in admin panel
- Add product_interaction_mode setting with options: order, manager, browser
- Add manager_username setting for Telegram manager contact
- Remove store_enabled setting, replaced with product_interaction_mode
- Create migration to automatically migrate store_enabled to product_interaction_mode
- Update Product.vue to handle all three interaction modes
- Update Dock.vue to show cart button only when product_interaction_mode is 'order'
- Rename 'Магазин' tab to 'Витрина' in admin panel
- Remove 'Разрешить покупки' option (now controlled via product_interaction_mode)
- Set default product_interaction_mode to 'browser'
- Update StoreDTO to remove enableStore field
- Update SettingsHandler to return product_interaction_mode instead of store_enabled

frontend/admin/src/App.vue

@@ -15,7 +15,7 @@
       </li>
 
       <li :class="{active: route.name === 'store'}">
-        <RouterLink :to="{name: 'store'}">Магазин</RouterLink>
+        <RouterLink :to="{name: 'store'}">Витрина</RouterLink>
       </li>
 
       <li :class="{active: route.name === 'texts'}">