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/Components/Molecules/AiResponseRenderer.razor

@@ -2,10 +2,12 @@
 @using NexusReader.UI.Shared.Services
 @using NexusReader.Application.DTOs.AI
 @using NexusReader.Application.DTOs.User
+@using Microsoft.Extensions.Logging
 @using System.Net.Http.Json
 @inject HttpClient Http
 @inject ILibraryStateService LibraryStateService
 @inject NavigationManager NavigationManager
+@inject ILogger<AiResponseRenderer> Logger
 
 <div class="message-row @(Message.Sender == "User" ? "user-row" : "ai-row")">
     <div class="message-avatar" aria-hidden="true">
@@ -200,12 +202,12 @@
             }
             else
             {
-                Console.WriteLine("[AiResponseRenderer] Purchase failed on server.");
+                Logger.LogWarning("[AiResponseRenderer] Purchase failed on server for book {BookId}.", _lockedBookId);
             }
         }
         catch (Exception ex)
         {
-            Console.WriteLine($"[AiResponseRenderer] Error processing purchase: {ex.Message}");
+            Logger.LogError(ex, "[AiResponseRenderer] Error processing purchase for book {BookId}.", _lockedBookId);
         }
         finally
         {