refactor: implement Result pattern for IdentityService and add CSS fallbacks

src/NexusReader.Web.New/Services/ServerIdentityService.cs

@@ -1,4 +1,5 @@
 using System.Security.Claims;
+using FluentResults;
 using Microsoft.AspNetCore.Identity;
 using Microsoft.EntityFrameworkCore;
 using NexusReader.Application.DTOs.User;
@@ -14,6 +15,8 @@ public class ServerIdentityService : IIdentityService
     private readonly IHttpContextAccessor _httpContextAccessor;
     private readonly IDbContextFactory<AppDbContext> _dbContextFactory;
 
+    public event Func<Task>? OnStateInvalidated;
+
     public ServerIdentityService(
         UserManager<NexusUser> userManager,
         IHttpContextAccessor httpContextAccessor,
@@ -24,24 +27,24 @@ public class ServerIdentityService : IIdentityService
         _dbContextFactory = dbContextFactory;
     }
 
-    public Task<bool> LoginAsync(string email, string password, bool rememberMe = false) 
+    public Task<Result> LoginAsync(string email, string password, bool rememberMe = false) 
         => throw new NotSupportedException("Use standard Identity endpoints for login on server.");
 
-    public Task LogoutAsync() 
+    public Task<Result> LogoutAsync() 
         => throw new NotSupportedException("Use standard Identity endpoints for logout on server.");
 
-    public Task<bool> RegisterAsync(string email, string password) 
+    public Task<Result> RegisterAsync(string email, string password) 
         => throw new NotSupportedException("Use standard Identity endpoints for registration on server.");
 
-    public Task<bool> RefreshTokenAsync() => Task.FromResult(true);
+    public Task<Result> RefreshTokenAsync() => Task.FromResult(Result.Ok());
 
-    public async Task<UserProfile?> GetProfileAsync()
+    public async Task<Result<UserProfile>> GetProfileAsync()
     {
         var user = _httpContextAccessor.HttpContext?.User;
-        if (user == null || !user.Identity?.IsAuthenticated == true) return null;
+        if (user == null || !user.Identity?.IsAuthenticated == true) return Result.Fail("Not authenticated.");
 
         var userId = user.FindFirstValue(ClaimTypes.NameIdentifier);
-        if (userId == null) return null;
+        if (userId == null) return Result.Fail("User ID not found.");
 
         using var dbContext = await _dbContextFactory.CreateDbContextAsync();
         
@@ -73,6 +76,6 @@ public class ServerIdentityService : IIdentityService
             ))
             .FirstOrDefaultAsync();
 
-        return profile;
+        return profile != null ? Result.Ok(profile) : Result.Fail("Profile not found.");
     }
 }