feat(ui): implement hub navigation, profile dashboard and fix auth sync loop

- Added MainHubLayout with glassmorphism sidebar
- Implemented Profile dashboard with learn metrics
- Added request deduplication and caching to IdentityService
- Fixed infinite redirect loop on /profile page
- Added dashboard navigation from reader
- Closes #26, Closes #27

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" />