fix: resolve PR #76 review recommendations

- Add ILogger<GetContextualRecommendationsQueryHandler> with structured logging
- Guard empty embedding text in VectorSearchStore (return empty vector, skip search)
- Benchmark vector search and embedding latency with Stopwatch (LogDebug/LogInfo)
- Refine EnsureCollectionExistsAsync: log creation events and non-fatal errors
- Replace all Console.WriteLine with ILogger in UI components (AiAssistantBubble,
  AiResponseRenderer, IntelligenceToolbar, SelectionAiPanel, Catalog, Intelligence, MyBooks)
- Create IRecommendationService abstraction + RecommendationService WASM implementation
- Register IRecommendationService in Web.Client DI
- Add ContextualRecommendationsWidget component with loading spinner and design tokens
- Add ContextualRecommendationsWidget to Dashboard.razor
- Update test constructor with ILogger mock for GetContextualRecommendationsQueryHandler

Closes review items: 2, 3, 4, 5, 6, 7, 8, 9, 10
Item 1 (unit tests) was already completed in previous session

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

@@ -4,10 +4,12 @@
 @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">
@@ -140,7 +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;