feat: Integrate ASP.NET Core Identity for user authentication

This commit integrates ASP.NET Core Identity into the application to enable user registration, login, and management. This lays the groundwork for securing data per user.

**Key Changes:**

*   **DbContext Configuration:**
    *   Modified `ApplicationDbContext.cs` to inherit from `IdentityDbContext<IdentityUser>`.
    *   Removed an unnecessary `using` statement from `ApplicationDbContext.cs`.

*   **Program.cs Setup:**
    *   Configured `AddDefaultIdentity<IdentityUser>` with `AddEntityFrameworkStores<ApplicationDbContext>()` to register Identity services.
    *   Ensured correct ordering of `UseAuthentication()` and `UseAuthorization()` middleware.
    *   Added `app.MapRazorPages()` to enable the Identity UI pages.
    *   Verified core package versions in `turf_tasker.csproj` for consistency across EF Core and Identity components (`8.0.6`).

*   **Identity UI:**
    *   Scaffolded ASP.NET Core Identity pages (Login, Register, Manage, etc.) to provide the user interface for authentication.
    *   Added a `_LoginPartial.cshtml` partial view to the `Views/Shared` folder.
    *   Rendered `_LoginPartial` in `Views/Shared/_Layout.cshtml` to display login/register/logout links in the navigation bar.

*   **Migrations:**
    *   Created and applied a new migration (`AddIdentitySchema`) to create the necessary ASP.NET Core Identity database tables (e.g., `AspNetUsers`, `AspNetRoles`).

Areas/Identity/Pages/Account/Manage/ChangePassword.cshtml.cs

@@ -0,0 +1,127 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+#nullable disable
+
+using System;
+using System.ComponentModel.DataAnnotations;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Identity;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+using Microsoft.Extensions.Logging;
+
+namespace turf_tasker.Areas.Identity.Pages.Account.Manage
+{
+    public class ChangePasswordModel : PageModel
+    {
+        private readonly UserManager<IdentityUser> _userManager;
+        private readonly SignInManager<IdentityUser> _signInManager;
+        private readonly ILogger<ChangePasswordModel> _logger;
+
+        public ChangePasswordModel(
+            UserManager<IdentityUser> userManager,
+            SignInManager<IdentityUser> signInManager,
+            ILogger<ChangePasswordModel> logger)
+        {
+            _userManager = userManager;
+            _signInManager = signInManager;
+            _logger = logger;
+        }
+
+        /// <summary>
+        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        [BindProperty]
+        public InputModel Input { get; set; }
+
+        /// <summary>
+        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        [TempData]
+        public string StatusMessage { get; set; }
+
+        /// <summary>
+        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        public class InputModel
+        {
+            /// <summary>
+            ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
+            ///     directly from your code. This API may change or be removed in future releases.
+            /// </summary>
+            [Required]
+            [DataType(DataType.Password)]
+            [Display(Name = "Current password")]
+            public string OldPassword { get; set; }
+
+            /// <summary>
+            ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
+            ///     directly from your code. This API may change or be removed in future releases.
+            /// </summary>
+            [Required]
+            [StringLength(100, ErrorMessage = "The {0} must be at least {2} and at max {1} characters long.", MinimumLength = 6)]
+            [DataType(DataType.Password)]
+            [Display(Name = "New password")]
+            public string NewPassword { get; set; }
+
+            /// <summary>
+            ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
+            ///     directly from your code. This API may change or be removed in future releases.
+            /// </summary>
+            [DataType(DataType.Password)]
+            [Display(Name = "Confirm new password")]
+            [Compare("NewPassword", ErrorMessage = "The new password and confirmation password do not match.")]
+            public string ConfirmPassword { get; set; }
+        }
+
+        public async Task<IActionResult> OnGetAsync()
+        {
+            var user = await _userManager.GetUserAsync(User);
+            if (user == null)
+            {
+                return NotFound($"Unable to load user with ID '{_userManager.GetUserId(User)}'.");
+            }
+
+            var hasPassword = await _userManager.HasPasswordAsync(user);
+            if (!hasPassword)
+            {
+                return RedirectToPage("./SetPassword");
+            }
+
+            return Page();
+        }
+
+        public async Task<IActionResult> OnPostAsync()
+        {
+            if (!ModelState.IsValid)
+            {
+                return Page();
+            }
+
+            var user = await _userManager.GetUserAsync(User);
+            if (user == null)
+            {
+                return NotFound($"Unable to load user with ID '{_userManager.GetUserId(User)}'.");
+            }
+
+            var changePasswordResult = await _userManager.ChangePasswordAsync(user, Input.OldPassword, Input.NewPassword);
+            if (!changePasswordResult.Succeeded)
+            {
+                foreach (var error in changePasswordResult.Errors)
+                {
+                    ModelState.AddModelError(string.Empty, error.Description);
+                }
+                return Page();
+            }
+
+            await _signInManager.RefreshSignInAsync(user);
+            _logger.LogInformation("User changed their password successfully.");
+            StatusMessage = "Your password has been changed.";
+
+            return RedirectToPage();
+        }
+    }
+}