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/Components/Molecules/SelectionAiPanel.razor

@@ -1,10 +1,12 @@
 @using NexusReader.UI.Shared.Services
 @using NexusReader.UI.Shared.Models
 @using NexusReader.Application.DTOs.AI
+@using Microsoft.Extensions.Logging
 @inject KnowledgeCoordinator Coordinator
 @inject IReaderInteractionService InteractionService
 @inject IQuizStateService QuizService
 @inject IJSRuntime JS
+@inject ILogger<SelectionAiPanel> Logger
 
 @if (IsVisible)
 {
@@ -64,7 +66,7 @@
 
     protected override void OnParametersSet()
     {
-        Console.WriteLine($"[SelectionAiPanel] Parameters set. SelectedText: {SelectedText.Length} chars, Coordinates: {Coordinates?.Top}");
+        Logger.LogDebug("[SelectionAiPanel] Parameters set. SelectedText: {Length} chars, Coordinates: {Top}", SelectedText.Length, Coordinates?.Top);
         
         if (Coordinates != _lastCoordinates)
         {
@@ -100,7 +102,7 @@
             }
             catch (Exception ex)
             {
-                Console.WriteLine($"[SelectionAiPanel] Error positioning toolbar: {ex.Message}");
+                Logger.LogWarning(ex, "[SelectionAiPanel] Error positioning toolbar.");
             }
         }
     }
@@ -133,7 +135,7 @@
         }
         catch (Exception ex)
         {
-            Console.WriteLine($"[SelectionAiPanel] Error requesting summary: {ex.Message}");
+            Logger.LogError(ex, "[SelectionAiPanel] Error requesting summary for block {BlockId}.", BlockId);
         }
         finally
         {
@@ -173,7 +175,7 @@
         }
         catch (Exception ex)
         {
-            Console.WriteLine($"[SelectionAiPanel] Error generating quiz: {ex.Message}");
+            Logger.LogError(ex, "[SelectionAiPanel] Error generating quiz for block {BlockId}.", BlockId);
         }
         finally
         {