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/_Layout.cshtml

@@ -108,6 +108,14 @@
                             <i class="bi bi-code-slash"></i> API
                         </a>
                     </li>
+                    <li class="nav-item dropdown">
+                        <a class="nav-link dropdown-toggle" href="#" id="userDropdown" role="button" data-bs-toggle="dropdown">
+                            <i class="bi bi-person-circle"></i> @User.Identity?.Name
+                        </a>
+                        <ul class="dropdown-menu dropdown-menu-end">
+                            <li><a class="dropdown-item" href="/Logout"><i class="bi bi-box-arrow-right"></i> Logout</a></li>
+                        </ul>
+                    </li>
                 </ul>
             </div>
         </div>