feat: add multi-channel subscription blocking UI and admin management

- Channel subscription blocking screen with per-channel status display
- Safe URL handling with protocol validation (no open redirect)
- Race-condition-safe subscription check with useRef guard
- Admin channel management page with create, toggle, delete
- Error handlers on all admin mutations with haptic feedback
- Proper type guard for channel subscription error detection
- Accessibility: aria-labels on icon-only buttons
- Localization: en, ru, zh, fa translations

src/pages/AdminPanel.tsx

@@ -482,6 +482,12 @@ export default function AdminPanel() {
       iconBg: 'bg-violet-500/20',
       iconColor: 'text-violet-400',
       items: [
+        {
+          to: '/admin/channel-subscriptions',
+          icon: <MegaphoneIcon />,
+          title: t('admin.nav.channelSubscriptions'),
+          description: t('admin.panel.channelSubscriptionsDesc'),
+        },
         {
           to: '/admin/settings',
           icon: <CogIcon />,