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/wwwroot/app.css

@@ -19,6 +19,22 @@
     --nexus-transition: 0.4s cubic-bezier(0.4, 0, 0.2, 1);
 }
 
+/* Global Glassmorphism with Fallback */
+.glass-panel {
+    background: rgba(20, 20, 20, 0.85); /* Darker fallback for readability */
+    border: 1px solid rgba(255, 255, 255, 0.05);
+    border-radius: 20px;
+    padding: 1.5rem;
+    transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
+}
+
+@supports (backdrop-filter: blur(10px)) {
+    .glass-panel {
+        background: rgba(255, 255, 255, 0.03);
+        backdrop-filter: blur(10px);
+    }
+}
+
 
 .theme-light {
     --nexus-bg: var(--nexus-paper);