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/Register.cshtml.cs

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+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+using Microsoft.AspNetCore.Authentication;
+using Microsoft.AspNetCore.Authentication.Cookies;
+using System.Security.Claims;
+using RR3CommunityServer.Services;
+using RR3CommunityServer.Models;
+
+namespace RR3CommunityServer.Pages;
+
+public class RegisterModel : PageModel
+{
+    private readonly IAuthService _authService;
+    private readonly ILogger<RegisterModel> _logger;
+
+    public RegisterModel(IAuthService authService, ILogger<RegisterModel> logger)
+    {
+        _authService = authService;
+        _logger = logger;
+    }
+
+    [BindProperty]
+    public string Username { get; set; } = string.Empty;
+
+    [BindProperty]
+    public string Email { get; set; } = string.Empty;
+
+    [BindProperty]
+    public string Password { get; set; } = string.Empty;
+
+    [BindProperty]
+    public string ConfirmPassword { get; set; } = string.Empty;
+
+    public string? ErrorMessage { get; set; }
+    public string? SuccessMessage { get; set; }
+
+    public void OnGet()
+    {
+        // If already logged in, redirect to admin panel
+        if (User.Identity?.IsAuthenticated == true)
+        {
+            Response.Redirect("/admin");
+        }
+    }
+
+    public async Task<IActionResult> OnPostAsync()
+    {
+        if (string.IsNullOrWhiteSpace(Username) || string.IsNullOrWhiteSpace(Email) || 
+            string.IsNullOrWhiteSpace(Password) || string.IsNullOrWhiteSpace(ConfirmPassword))
+        {
+            ErrorMessage = "All fields are required";
+            return Page();
+        }
+
+        var registerRequest = new RegisterRequest
+        {
+            Username = Username,
+            Email = Email,
+            Password = Password,
+            ConfirmPassword = ConfirmPassword
+        };
+
+        var (success, token, error) = await _authService.RegisterAsync(registerRequest);
+
+        if (!success || string.IsNullOrEmpty(token))
+        {
+            ErrorMessage = error ?? "Registration failed";
+            _logger.LogWarning("Failed registration attempt for: {Username}", Username);
+            return Page();
+        }
+
+        _logger.LogInformation("New account registered: {Username} ({Email})", Username, Email);
+
+        // Auto-login after registration
+        var loginRequest = new LoginRequest
+        {
+            UsernameOrEmail = Username,
+            Password = Password
+        };
+
+        var (loginSuccess, response, loginError) = await _authService.LoginAsync(loginRequest);
+
+        if (loginSuccess && response != null)
+        {
+            var claims = new List<Claim>
+            {
+                new Claim(ClaimTypes.NameIdentifier, response.AccountId.ToString()),
+                new Claim(ClaimTypes.Name, response.Username),
+                new Claim(ClaimTypes.Email, response.Email)
+            };
+
+            var claimsIdentity = new ClaimsIdentity(claims, CookieAuthenticationDefaults.AuthenticationScheme);
+            var authProperties = new AuthenticationProperties
+            {
+                IsPersistent = true,
+                ExpiresUtc = response.ExpiresAt
+            };
+
+            await HttpContext.SignInAsync(
+                CookieAuthenticationDefaults.AuthenticationScheme,
+                new ClaimsPrincipal(claimsIdentity),
+                authProperties);
+
+            return RedirectToPage("/Admin");
+        }
+
+        SuccessMessage = "Account created successfully! Please login.";
+        return RedirectToPage("/Login");
+    }
+}