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.Maui/Main.razor

@@ -3,7 +3,7 @@
 
 <Router AppAssembly="@typeof(NexusReader.UI.Shared._Imports).Assembly">
     <Found Context="routeData">
-        <AuthorizeRouteView RouteData="@routeData" DefaultLayout="@typeof(NexusReader.UI.Shared.Layout.MainLayout)">
+        <AuthorizeRouteView RouteData="@routeData" DefaultLayout="@typeof(NexusReader.UI.Shared.Layout.MainHubLayout)">
             <NotAuthorized>
                 <RedirectToLogin />
             </NotAuthorized>
@@ -11,7 +11,7 @@
         <FocusOnNavigate RouteData="@routeData" Selector="h1" />
     </Found>
     <NotFound>
-        <LayoutView Layout="@typeof(NexusReader.UI.Shared.Layout.MainLayout)">
+        <LayoutView Layout="@typeof(NexusReader.UI.Shared.Layout.MainHubLayout)">
             <p role="alert">Sorry, there's nothing at this address.</p>
         </LayoutView>
     </NotFound>