feat: add plan assignment to user management

- Added plan dropdown to user edit modal
- Shows current plan with highlighted card
- Allows admin to assign/change user's subscription plan
- Loads user's current plan when opening edit modal
- Updates plan via setUserPlan service
- Visual feedback with success/error messages

Admins can now:
- View user's current subscription plan
- Assign users to different plans (Starter, Bronze, Silver, Gold)
- See plan details (credits/month, price) in dropdown

This completes the subscription plan system!

src/app/admin/users/page.tsx

@@ -3,6 +3,7 @@ import { cookies } from 'next/headers';
 import { redirect } from 'next/navigation';
 import { checkIsAdmin } from '@/services/track-api-usage';
 import { getAllUsersWithCredits } from '@/services/admin-credit-service';
+import { getAllPlans } from '@/services/subscription-service';
 import Link from 'next/link';
 import { ChevronLeft, Users, Coins, TrendingUp, TrendingDown } from 'lucide-react';
 import UserManagementClient from '@/components/UserManagementClient';
@@ -23,6 +24,9 @@ export default async function AdminUsersPage() {
     // Fetch all users with credits
     const users = await getAllUsersWithCredits();
 
+    // Fetch all plans
+    const plans = await getAllPlans();
+
     // Calculate statistics
     const totalUsers = users.length;
     const totalCreditsInCirculation = users.reduce((sum, u) => sum + u.balance, 0);
@@ -91,7 +95,7 @@ export default async function AdminUsersPage() {
                 </div>
 
                 {/* User Management Table */}
-                <UserManagementClient initialUsers={users} />
+                <UserManagementClient initialUsers={users} plans={plans} />
             </div>
         </main>
     );