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

tests/NexusReader.Application.Tests/Queries/GetContextualRecommendationsQueryTests.cs

@@ -4,6 +4,7 @@ using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using FluentAssertions;
+using Microsoft.Extensions.Logging;
 using Moq;
 using NexusReader.Application.Abstractions.Persistence;
 using NexusReader.Application.Queries.Recommendations;
@@ -17,6 +18,7 @@ public class GetContextualRecommendationsQueryTests
     private readonly Mock<IUserReadingStateStore> _readingStateStoreMock;
     private readonly Mock<IUserLibraryStore> _libraryStoreMock;
     private readonly Mock<IVectorSearchStore> _vectorSearchStoreMock;
+    private readonly Mock<ILogger<GetContextualRecommendationsQueryHandler>> _loggerMock;
     private readonly GetContextualRecommendationsQueryHandler _handler;
 
     public GetContextualRecommendationsQueryTests()
@@ -24,11 +26,13 @@ public class GetContextualRecommendationsQueryTests
         _readingStateStoreMock = new Mock<IUserReadingStateStore>();
         _libraryStoreMock = new Mock<IUserLibraryStore>();
         _vectorSearchStoreMock = new Mock<IVectorSearchStore>();
+        _loggerMock = new Mock<ILogger<GetContextualRecommendationsQueryHandler>>();
 
         _handler = new GetContextualRecommendationsQueryHandler(
             _readingStateStoreMock.Object,
             _libraryStoreMock.Object,
-            _vectorSearchStoreMock.Object
+            _vectorSearchStoreMock.Object,
+            _loggerMock.Object
         );
     }