feat(ai-ux): deduplicate AI queries, handle ServiceUnavailable retries, and optimize reader canvas graph prerendering (#44)

This Pull Request encapsulates all outstanding AI, Blazor InteractiveAuto lifecycle, pgvector, and Firefox authorization/session compatibility fixes.

### Key Accomplishments:
1. **Concurrent Request Deduplication (Option B):** Implemented a thread-safe active task registry in `KnowledgeService` that groups concurrent graph extraction queries for the same content, preventing duplicate AI calls completely.
2. **Resilience Strategy for Downstream Demands:** Extended the `ai-retry` resilience pipeline to automatically intercept and retry on temporary Google API `503 ServiceUnavailable` / `high demand` spikes.
3. **Interactive Graph Generation Guard (Option A):** Prevented server-side prerender-phase graph requests in the reader canvas component.
4. **Firefox Compatibility & Cookie Handler:** Implemented an authentication endpoint and hybrid hidden-form submission flow to solve login, registration, and logout redirections and cookies securely.
5. **Autoscrolling & Graph Exclusions:** Added concept-to-block smooth scrolling, active block badging, and filtered out markdown code blocks from being extracted as nodes.

All unit tests compiled and passed 100% cleanly.

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #44
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

src/NexusReader.Application/Mappings/MappingConfig.cs

@@ -13,7 +13,8 @@ public static class MappingConfig
         var config = TypeAdapterConfig.GlobalSettings;
         
         config.NewConfig<NexusUser, UserProfileDto>();
-            // Roles are mapped manually in queries due to Identity structure
+        config.NewConfig<Ebook, LastReadBookDto>()
+            .Map(dest => dest.Description, src => src.Description);
 
         services.AddSingleton(config);
         services.AddScoped<IMapper, ServiceMapper>();
@@ -21,3 +22,4 @@ public static class MappingConfig
         return services;
     }
 }
+