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.Domain/Entities/Ebook.cs

@@ -15,8 +15,11 @@ public class Ebook
     [MaxLength(255)]
     public string Title { get; set; } = string.Empty;
 
-    [MaxLength(255)]
-    public string Author { get; set; } = "Unknown";
+    [Required]
+    public int AuthorId { get; set; }
+
+    [ForeignKey(nameof(AuthorId))]
+    public virtual Author Author { get; set; } = null!;
 
     [Required]
     public string FilePath { get; set; } = string.Empty;
@@ -30,6 +33,13 @@ public class Ebook
     public DateTime AddedDate { get; set; } = DateTime.UtcNow;
 
     public DateTime? LastReadDate { get; set; }
+    
+    public double Progress { get; set; } = 0;
+
+    [MaxLength(255)]
+    public string? LastChapter { get; set; }
+
+    public int LastChapterIndex { get; set; } = 0;
 
     // Relationship to NexusUser
     [Required]