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.Data/Persistence/DbInitializer.cs

@@ -78,6 +78,33 @@ public static class DbInitializer
                 await dbContext.SaveChangesAsync();
 
                 Console.WriteLine($"[Seeder] Admin user created successfully: {adminEmail}");
+
+                // Seed Sample Author
+                var author = await dbContext.Authors.FirstOrDefaultAsync(a => a.Name == "Giorgio Vasari");
+                if (author == null)
+                {
+                    author = new Author { Name = "Giorgio Vasari" };
+                    dbContext.Authors.Add(author);
+                    await dbContext.SaveChangesAsync();
+                }
+
+                // Seed Sample Ebook
+                if (!dbContext.Ebooks.Any(e => e.UserId == adminUser.Id))
+                {
+                    dbContext.Ebooks.Add(new Ebook
+                    {
+                        Title = "Lives of the Most Excellent Painters, Sculptors, and Architects",
+                        AuthorId = author.Id,
+                        UserId = adminUser.Id,
+                        FilePath = "wwwroot/assets/book.epub",
+                        AddedDate = DateTime.UtcNow,
+                        LastReadDate = DateTime.UtcNow,
+                        Progress = 0,
+                        LastChapter = "Introduction"
+                    });
+                    await dbContext.SaveChangesAsync();
+                    Console.WriteLine("[Seeder] Sample book seeded for admin.");
+                }
             }
             else
             {