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.UI.Shared/Components/Organisms/BookIngestionModal.razor

@@ -55,6 +55,10 @@
                                     <label>Author</label>
                                     <input type="text" class="form-input" @bind="Metadata.Author" placeholder="Enter author name" />
                                 </div>
+                                <div class="form-group">
+                                    <label>Description</label>
+                                    <textarea class="form-input" @bind="Metadata.Description" placeholder="Book description" rows="3"></textarea>
+                                </div>
                             </div>
                         </div>
 
@@ -205,7 +209,8 @@
                 Metadata.Title,
                 Metadata.Author,
                 Metadata.CoverImage != null ? Convert.ToBase64String(Metadata.CoverImage) : null,
-                Convert.ToBase64String(_epubBytes)
+                Convert.ToBase64String(_epubBytes),
+                Metadata.Description
             );
 
             var response = await Http.PostAsJsonAsync("api/library/ingest", request);