feat(ui): Hub Navigation, Profile Dashboard and Auth Stability Fixes (#31)

This PR implements the Hub Navigation system and the Profile Dashboard, while resolving critical session synchronization issues.

### Key Changes
- **Hub Navigation**: Introduced `MainHubLayout` with a premium glassmorphism sidebar, providing access to Dashboard, Library, Concepts Map, and Profile.
- **Profile Dashboard**: Implemented a high-fidelity Profile page (#27) with learning metrics, AI token usage tracking, and system rank visualization.
- **Stability Fixes**:
    - Resolved an infinite network loop on the `/profile` page by implementing request deduplication and in-memory caching in `IdentityService`.
    - Added environment-aware guards to prevent illegal JavaScript interop calls during server-side prerendering.
    - Implemented automatic session invalidation on `401 Unauthorized` responses to handle stale authentication states gracefully.
- **Reader Integration**: Added a "Return to Dashboard" option in the reader toolbar (#26).

Closes #26
Closes #27

Reviewed-on: #31
Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Co-committed-by: Marek Jasiński <jasins.marek@gmail.com>

src/NexusReader.Infrastructure/Services/EpubService.cs

@@ -4,15 +4,24 @@ using FluentResults;
 using NexusReader.Application.Abstractions.Services;
 using NexusReader.Application.Queries.Reader;
 using VersOne.Epub;
+using Microsoft.EntityFrameworkCore;
+using NexusReader.Data.Persistence;
+using NexusReader.Domain.Entities;
 
 namespace NexusReader.Infrastructure.Services;
 
 public class EpubService : IEpubService
 {
+    private readonly IDbContextFactory<AppDbContext> _dbContextFactory;
     private const string EpubPath = "wwwroot/assets/book.epub";
     private const int WordThreshold = 1000;
 
-    public async Task<Result<ReaderPageViewModel>> GetEpubContentAsync(int chapterIndex)
+    public EpubService(IDbContextFactory<AppDbContext> dbContextFactory)
+    {
+        _dbContextFactory = dbContextFactory;
+    }
+
+    public async Task<Result<ReaderPageViewModel>> GetEpubContentAsync(int chapterIndex, string? userId = null)
     {
         try
         {
@@ -100,7 +109,29 @@ public class EpubService : IEpubService
                 blocks.Add(CreateAiTrigger($"trigger-{blockCounter++}"));
             }
 
-            return Result.Ok(new ReaderPageViewModel(blocks, chapterIndex, readingOrder.Count, chapterTitle));
+            // Find the EbookId from DB for this file AND this user
+            using var context = await _dbContextFactory.CreateDbContextAsync();
+            var ebook = await context.Ebooks
+                .Where(e => e.FilePath.Contains("book.epub") && (userId == null || e.UserId == userId))
+                .FirstOrDefaultAsync();
+
+            // Auto-provision if not found for this user (convenience for dev)
+            if (ebook == null && !string.IsNullOrEmpty(userId))
+            {
+                var author = await context.Authors.FirstOrDefaultAsync() ?? new Author { Name = "Unknown Author" };
+                ebook = new Ebook
+                {
+                    Title = "Lives of the Most Excellent Painters, Sculptors, and Architects",
+                    FilePath = "wwwroot/assets/book.epub",
+                    UserId = userId,
+                    Author = author,
+                    TenantId = "global"
+                };
+                context.Ebooks.Add(ebook);
+                await context.SaveChangesAsync();
+            }
+
+            return Result.Ok(new ReaderPageViewModel(blocks, chapterIndex, readingOrder.Count, chapterTitle, ebook?.Id ?? Guid.Empty));
         }
         catch (Exception ex)
         {