feat(admin): centralize subscriptions under /admin?tab=subscriptions

Replaced the dedicated /admin/subscriptions page with a redirect to the central admin panel’s subscriptions tab. Removed session checks, imports, and the AdminSubscriptionDashboard since routing and access are now handled centrally. This streamlines admin UX and keeps access control unified.

package.json

@@ -53,6 +53,7 @@
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "cobe": "^0.6.5",
+    "date-fns": "^4.1.0",
     "jsonwebtoken": "^9.0.2",
     "lucide-react": "^0.540.0",
     "mongoose": "^8.19.3",