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.UI.Shared/Pages/Account/Register.razor

@@ -81,17 +81,32 @@
 
         try
         {
-            var success = await IdentityService.RegisterAsync(_registerModel.Email, _registerModel.Password);
-            if (success)
+            var regResult = await IdentityService.RegisterAsync(_registerModel.Email, _registerModel.Password);
+            if (regResult.IsSuccess)
             {
-                var loginSuccess = await IdentityService.LoginAsync(_registerModel.Email, _registerModel.Password);
-                if (loginSuccess) NavigationManager.NavigateTo("/");
-                else NavigationManager.NavigateTo("/account/login");
+                var loginResult = await IdentityService.LoginAsync(_registerModel.Email, _registerModel.Password);
+                if (loginResult.IsSuccess) 
+                {
+                    NavigationManager.NavigateTo("/");
+                }
+                else 
+                {
+                    NavigationManager.NavigateTo("/account/login");
+                }
+            }
+            else 
+            { 
+                _errorMessage = regResult.Errors.FirstOrDefault()?.Message ?? "Rejestracja nie powiodła się."; 
             }
-            else { _errorMessage = "Rejestracja nie powiodła się."; }
         }
-        catch (Exception) { _errorMessage = "Wystąpił błąd podczas rejestracji."; }
-        finally { _isSubmitting = false; }
+        catch (Exception) 
+        { 
+            _errorMessage = "Wystąpił błąd podczas rejestracji."; 
+        }
+        finally 
+        { 
+            _isSubmitting = false; 
+        }
     }
 
     public class RegisterModel