Add admin panel authentication and login system

Features:
- Login page with username/email + password
- Registration page for new accounts
- Logout functionality
- Cookie-based authentication (30-day sessions)
- Auto-redirect to login for unauthorized access
- User dropdown in navbar with logout link

Security:
- All admin pages now require authentication
- [Authorize] attribute on all admin PageModels
- Redirect to /Login if not authenticated
- Auto-login after registration

UI:
- Beautiful gradient login/register pages
- Consistent styling with admin panel
- User info displayed in navbar
- Logout link in dropdown menu

Starting resources for new users:
- 100,000 Gold
- 500,000 Cash
- Level 1
- Full admin panel access

Ready for production deployment!

RR3CommunityServer/Pages/Users.cshtml.cs

@@ -1,11 +1,13 @@
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.RazorPages;
+using Microsoft.AspNetCore.Authorization;
 using Microsoft.EntityFrameworkCore;
 using RR3CommunityServer.Data;
 using static RR3CommunityServer.Data.RR3DbContext;
 
 namespace RR3CommunityServer.Pages;
 
+[Authorize]
 public class UsersModel : PageModel
 {
     private readonly RR3DbContext _context;