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/IntelligenceToolbar.razor

@@ -1,10 +1,13 @@
 @using NexusReader.UI.Shared.Services
 @using NexusReader.Application.Abstractions.Services
+@using Microsoft.Extensions.Logging
+@using System.Linq
 @inject IFocusModeService FocusMode
 @inject IIdentityService IdentityService
 @inject NavigationManager NavigationManager
 @inject IThemeService ThemeService
 @inject IKnowledgeService KnowledgeService
+@inject ILogger<IntelligenceToolbar> Logger
 @implements IDisposable
 
 <aside class="intelligence-toolbar">
@@ -51,11 +54,15 @@
 
     private async Task HandleClearCache()
     {
-        Console.WriteLine("[IntelligenceToolbar] Requesting cache clear...");
+        Logger.LogInformation("[IntelligenceToolbar] Requesting cache clear...");
         var result = await KnowledgeService.ClearCacheAsync();
         if (result.IsSuccess)
         {
-            Console.WriteLine("[IntelligenceToolbar] Cache cleared successfully!");
+            Logger.LogInformation("[IntelligenceToolbar] Cache cleared successfully.");
+        }
+        else
+        {
+            Logger.LogWarning("[IntelligenceToolbar] Cache clear failed: {Errors}", string.Join("; ", result.Errors.Select(e => e.Message)));
         }
     }