feat: add ButtonsTab for per-section button style customization

Visual editor in admin settings for configuring Telegram button colors
and custom emoji IDs per menu section. Includes API client, draft state
management, save/reset, and translations for ru/en/fa/zh locales.

src/components/admin/constants.ts

@@ -21,6 +21,7 @@ export const MENU_SECTIONS: MenuSection[] = [
       { id: 'branding', iconType: null },
       { id: 'theme', iconType: null },
       { id: 'analytics', iconType: null },
+      { id: 'buttons', iconType: null },
     ],
   },
   {