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/Components/Molecules/IntelligenceToolbar.razor

@@ -7,8 +7,8 @@
 
 <aside class="intelligence-toolbar">
     <div class="toolbar-top">
-        <button class="toolbar-item" title="Back">
-            <NexusIcon Name="play" Size="20" Class="rotate-180" />
+        <button class="toolbar-item" @onclick='() => NavigationManager.NavigateTo("/")' title="Back to Dashboard">
+            <NexusIcon Name="arrow-left" Size="20" />
         </button>
         <button class="toolbar-item active" title="Chat">
             <NexusIcon Name="message-square" Size="20" />
@@ -35,8 +35,8 @@
                 @onclick="FocusMode.ToggleAsync" title="Focus Mode (F)">
             <NexusIcon Name="target" Size="20" />
         </button>
-        <button class="toolbar-item" title="Global Settings">
-            <NexusIcon Name="settings" Size="20" />
+        <button class="toolbar-item" @onclick='() => NavigationManager.NavigateTo("/")' title="Global Hub">
+            <NexusIcon Name="layers" Size="20" />
         </button>
         <button class="toolbar-item logout-item" @onclick="HandleLogout" title="Exit">
             <NexusIcon Name="log-out" Size="20" />