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/Commands/Library/IngestEbookCommand.cs

@@ -9,6 +9,7 @@ namespace NexusReader.Application.Commands.Library;
 /// <param name="AuthorName">The name of the author.</param>
 /// <param name="CoverImage">The raw bytes of the cover image (optional).</param>
 /// <param name="EpubData">The raw bytes of the EPUB file.</param>
+/// <param name="Description">The description or summary of the book (optional).</param>
 /// <param name="UserId">The ID of the user owning the book.</param>
 /// <param name="TenantId">The tenant ID for multi-tenant isolation. Defaults to "global" for single-tenant or default usage.</param>
 public record IngestEbookCommand(
@@ -16,6 +17,7 @@ public record IngestEbookCommand(
     string AuthorName,
     byte[]? CoverImage,
     byte[] EpubData,
+    string? Description,
     string UserId,
     string TenantId = "global"
 ) : ICommand<Guid>;

src/NexusReader.Application/Commands/Library/IngestEbookCommandHandler.cs

@@ -63,6 +63,7 @@ public class IngestEbookCommandHandler : IRequestHandler<IngestEbookCommand, Res
                 Author = author,
                 FilePath = epubPath,
                 CoverUrl = coverUrl,
+                Description = request.Description,
                 UserId = request.UserId,
                 TenantId = request.TenantId,
                 AddedDate = DateTime.UtcNow

src/NexusReader.Application/Commands/Library/IngestEbookRequest.cs

@@ -4,5 +4,6 @@ public record IngestEbookRequest(
     string Title,
     string AuthorName,
     string? CoverImageBase64,
-    string EpubDataBase64
+    string EpubDataBase64,
+    string? Description = null
 );