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/Index.cshtml.cs

@@ -0,0 +1,118 @@
+// 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.Text.Encodings.Web;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Identity;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+
+namespace turf_tasker.Areas.Identity.Pages.Account.Manage
+{
+    public class IndexModel : PageModel
+    {
+        private readonly UserManager<IdentityUser> _userManager;
+        private readonly SignInManager<IdentityUser> _signInManager;
+
+        public IndexModel(
+            UserManager<IdentityUser> userManager,
+            SignInManager<IdentityUser> signInManager)
+        {
+            _userManager = userManager;
+            _signInManager = signInManager;
+        }
+
+        /// <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 string Username { 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>
+        [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>
+        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>
+            [Phone]
+            [Display(Name = "Phone number")]
+            public string PhoneNumber { get; set; }
+        }
+
+        private async Task LoadAsync(IdentityUser user)
+        {
+            var userName = await _userManager.GetUserNameAsync(user);
+            var phoneNumber = await _userManager.GetPhoneNumberAsync(user);
+
+            Username = userName;
+
+            Input = new InputModel
+            {
+                PhoneNumber = phoneNumber
+            };
+        }
+
+        public async Task<IActionResult> OnGetAsync()
+        {
+            var user = await _userManager.GetUserAsync(User);
+            if (user == null)
+            {
+                return NotFound($"Unable to load user with ID '{_userManager.GetUserId(User)}'.");
+            }
+
+            await LoadAsync(user);
+            return Page();
+        }
+
+        public async Task<IActionResult> OnPostAsync()
+        {
+            var user = await _userManager.GetUserAsync(User);
+            if (user == null)
+            {
+                return NotFound($"Unable to load user with ID '{_userManager.GetUserId(User)}'.");
+            }
+
+            if (!ModelState.IsValid)
+            {
+                await LoadAsync(user);
+                return Page();
+            }
+
+            var phoneNumber = await _userManager.GetPhoneNumberAsync(user);
+            if (Input.PhoneNumber != phoneNumber)
+            {
+                var setPhoneResult = await _userManager.SetPhoneNumberAsync(user, Input.PhoneNumber);
+                if (!setPhoneResult.Succeeded)
+                {
+                    StatusMessage = "Unexpected error when trying to set phone number.";
+                    return RedirectToPage();
+                }
+            }
+
+            await _signInManager.RefreshSignInAsync(user);
+            StatusMessage = "Your profile has been updated";
+            return RedirectToPage();
+        }
+    }
+}