feat(recommendations): implement contextual recommendation engine (#76)

Resolves #75

### Description
This pull request implements a smart, Native AOT-compliant contextual recommendation engine for the desktop dashboard to drive user retention and cross-book monetization.

### Key Changes
1. **Application Layer**:
   - Declared `IUserReadingStateStore` interface to decouple reading state discovery.
   - Added `IVectorSearchStore.SearchGlobalExcludeAsync(...)` to abstract semantic similarity searches with exclusions.
   - Defined `GetContextualRecommendationsQuery` and response DTOs (`ContextualRecommendationResponse`, `RecommendationDto`).
2. **Infrastructure Layer**:
   - Implemented `UserReadingStateStore` using EF Core with DbContext pooling.
   - Implemented `SearchGlobalExcludeAsync` in `VectorSearchStore` to construct gRPC Qdrant filters (excluding the active book ID) and fetch `bookTitle` and `chapterTitle` from point payloads.
   - Implemented `GetContextualRecommendationsQueryHandler` using clean abstractions.
3. **Web & Serialization Layer**:
   - Mapped `GET /api/recommendations` endpoint.
   - Registered types in `AppJsonContext.cs` for AOT-compliant JSON serialization.
4. **Verification**:
   - Added complete unit test coverage in `GetContextualRecommendationsQueryTests.cs`. All 30 unit tests pass.

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #76
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

src/NexusReader.UI.Shared/Pages/MyBooks.razor

@@ -1,11 +1,15 @@
 @page "/my-books"
 @attribute [Authorize]
+@implements IDisposable
 @using NexusReader.UI.Shared.Components.Organisms
 @using NexusReader.Application.DTOs.User
 @using NexusReader.UI.Shared.Services
+@using Microsoft.Extensions.Logging
 @using System.Net.Http.Json
 @inject HttpClient Http
 @inject IReaderNavigationService ReaderNavigation
+@inject ILibraryStateService LibraryStateService
+@inject ILogger<MyBooks> Logger
 
 <div class="my-books-page">
     <header class="my-books-header">
@@ -108,6 +112,11 @@
     private bool _isLoading = true;
     private List<LastReadBookDto>? _books;
 
+    protected override void OnInitialized()
+    {
+        LibraryStateService.OnBooksChanged += HandleBooksChanged;
+    }
+
     protected override async Task OnAfterRenderAsync(bool firstRender)
     {
         if (firstRender)
@@ -116,6 +125,11 @@
         }
     }
 
+    private void HandleBooksChanged()
+    {
+        _ = InvokeAsync(LoadBooksAsync);
+    }
+
     private async Task LoadBooksAsync()
     {
         _isLoading = true;
@@ -128,7 +142,7 @@
         }
         catch (Exception ex)
         {
-            Console.WriteLine($"[MyBooks] Failed to load books: {ex.Message}");
+            Logger.LogError(ex, "[MyBooks] Failed to load books.");
             if (OperatingSystem.IsBrowser())
             {
                 _isLoading = false;
@@ -149,4 +163,9 @@
     {
         ReaderNavigation.NavigateToBook(bookId);
     }
+
+    public void Dispose()
+    {
+        LibraryStateService.OnBooksChanged -= HandleBooksChanged;
+    }
 }