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/Logout.cshtml.cs

@@ -0,0 +1,27 @@
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+using Microsoft.AspNetCore.Authentication;
+using Microsoft.AspNetCore.Authentication.Cookies;
+
+namespace RR3CommunityServer.Pages;
+
+public class LogoutModel : PageModel
+{
+    private readonly ILogger<LogoutModel> _logger;
+
+    public LogoutModel(ILogger<LogoutModel> logger)
+    {
+        _logger = logger;
+    }
+
+    public async Task<IActionResult> OnGetAsync()
+    {
+        var username = User.Identity?.Name ?? "Unknown";
+        
+        await HttpContext.SignOutAsync(CookieAuthenticationDefaults.AuthenticationScheme);
+        
+        _logger.LogInformation("User logged out: {Username}", username);
+        
+        return RedirectToPage("/Login");
+    }
+}