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/AppDbContext.cs

@@ -24,6 +24,7 @@ public class AppDbContext : IdentityDbContext<NexusUser>
     public DbSet<Ebook> Ebooks => Set<Ebook>();
     public DbSet<QuizResult> QuizResults => Set<QuizResult>();
     public DbSet<SubscriptionPlan> SubscriptionPlans => Set<SubscriptionPlan>();
+    public DbSet<Author> Authors => Set<Author>();
 
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
@@ -88,6 +89,11 @@ public class AppDbContext : IdentityDbContext<NexusUser>
                 .WithMany(u => u.Ebooks)
                 .HasForeignKey(e => e.UserId)
                 .OnDelete(DeleteBehavior.Cascade);
+
+            entity.HasOne(e => e.Author)
+                .WithMany(a => a.Ebooks)
+                .HasForeignKey(e => e.AuthorId)
+                .OnDelete(DeleteBehavior.Restrict);
             
             entity.HasIndex(e => e.TenantId);
         });