feat(ui): implement premium mobile-first reader layout with three-tab bottom navigation and assistant FAB (#57)

This pull request delivers a comprehensive mobile-first user experience overhaul for the NexusReader SaaS platform, specifically optimizing the Reader Canvas, D3.js Knowledge Graph representation, Dashboard card grid layout, and the application-wide navigation shell on mobile viewports (< 768px).

### Key Enhancements:
1. **Interactive Three-Tab Bottom Navigation**: Added premium, frosted glassy bottom-bar for mobile viewports to switch between standard reading, D3.js graph visual workspace, and structural concept reviews/quizzes.
2. **Contextual Floating Action Button (FAB)**: Introduced the AI Assistant FAB on mobile canvas layout with responsive animation, state-synchronization to trigger corresponding quiz views, and pulsing badge notification when new quizzes are dynamically generated.
3. **Adaptive D3.js Simulation & Rendering**: Integrated `setMobileMode(isMobile)` logic inside the D3 simulation engine, optimizing forces, rendering compact glyph pills, and installing auto-resize observers.
4. **Architectural & Native AOT Cleanliness**: Clean separation of layouts, fully scoped CSS configurations, functional-safe event orchestration inside `IReaderInteractionService`, and zero compiler errors.

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #57
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

src/NexusReader.UI.Shared/Components/Organisms/ReaderCanvas.razor

@@ -13,6 +13,8 @@
 @inject IReaderInteractionService InteractionService
 @inject ISyncService SyncService
 @inject AuthenticationStateProvider AuthStateProvider
+@inject IQuizStateService QuizService
+@inject IPlatformService PlatformService
 @inject ILogger<ReaderCanvas> Logger
 
 <div class="reader-canvas @(ThemeService.IsLightMode ? "theme-light" : "theme-dark")">
@@ -53,6 +55,17 @@
         BlockId="@_selectedBlockId" 
         Coordinates="@_selectionCoords"
         FullPageContent="@GetFullPageContent()" />
+
+    @if (_isMobile)
+    {
+        <button class="nexus-mobile-assistant-fab @(QuizService.HasNewQuiz ? "has-new-quiz" : "")" @onclick="HandleAssistantFabClick" aria-label="Asystent AI">
+            <NexusIcon Name="robot" Size="24" Class="neon-glow" />
+            @if (QuizService.HasNewQuiz)
+            {
+                <span class="fab-badge"></span>
+            }
+        </button>
+    }
 </div>
 
 @code {
@@ -68,17 +81,29 @@
     private ElementReference _containerRef;
     private bool _isInteractive;
     private string? _currentActiveBlockId;
+    private bool _isMobile = false;
 
     protected override async Task OnInitializedAsync()
     {
         await Coordinator.ClearAsync();
         ThemeService.OnThemeChanged += HandleUpdate;
         NavigationService.OnNavigationChanged += OnNavigationChanged;
+        QuizService.OnQuizUpdated += HandleUpdate;
 
         InteractionService.OnScrollToBlockRequested += HandleScrollRequested;
         InteractionService.OnHighlightBlockRequested += HandleHighlightRequested;
         InteractionService.OnTextSelected += HandleTextSelected;
         SyncService.OnProgressReceived += HandleSyncProgressReceived;
+
+        var context = PlatformService.GetDeviceContext();
+        if (context.IsSuccess)
+        {
+            _isMobile = context.Value.DeviceType switch
+            {
+                DeviceType.Phone or DeviceType.Tablet => true,
+                _ => false
+            };
+        }
     }
 
     protected override async Task OnParametersSetAsync()
@@ -286,10 +311,16 @@
 
     private Task HandleUpdate() => InvokeAsync(StateHasChanged);
 
+    private async Task HandleAssistantFabClick()
+    {
+        await InteractionService.RequestAssistant();
+    }
+
     public void Dispose()
     {
         ThemeService.OnThemeChanged -= HandleUpdate;
         NavigationService.OnNavigationChanged -= OnNavigationChanged;
+        QuizService.OnQuizUpdated -= HandleUpdate;
 
         InteractionService.OnScrollToBlockRequested -= HandleScrollRequested;
         InteractionService.OnHighlightBlockRequested -= HandleHighlightRequested;