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/Services/IReaderNavigationService.cs

@@ -18,4 +18,9 @@ public interface IReaderNavigationService
     /// Navigates to the reader for a specific book and records the current ebook ID.
     /// </summary>
     void NavigateToBook(Guid bookId);
+
+    /// <summary>
+    /// Sets the active book context (ID and optional chapter) without triggering browser routing.
+    /// </summary>
+    void SetBook(Guid bookId, int chapterIndex = 0);
 }

src/NexusReader.UI.Shared/Services/KnowledgeCoordinator.cs

@@ -43,8 +43,34 @@ public sealed partial class KnowledgeCoordinator : IDisposable
 
     private async Task HandleNodeSelected(string nodeId)
     {
-        await _interactionService.RequestScrollToBlock(nodeId);
-        await _interactionService.RequestHighlightBlock(nodeId);
+        string? targetBlockId = nodeId;
+        
+        var graph = _graphService.CurrentGraphData;
+        if (graph != null)
+        {
+            var selectedNode = graph.Nodes.FirstOrDefault(n => n.Id == nodeId);
+            if (selectedNode != null && selectedNode.Group == "concept")
+            {
+                // Look for connected block nodes (group: "current") in the links
+                var connectedLinks = graph.Links.Where(l => l.Source == nodeId || l.Target == nodeId).ToList();
+                foreach (var link in connectedLinks)
+                {
+                    var otherId = link.Source == nodeId ? link.Target : link.Source;
+                    var otherNode = graph.Nodes.FirstOrDefault(n => n.Id == otherId);
+                    if (otherNode != null && otherNode.Group == "current")
+                    {
+                        targetBlockId = otherId;
+                        break;
+                    }
+                }
+            }
+        }
+
+        if (!string.IsNullOrEmpty(targetBlockId))
+        {
+            await _interactionService.RequestScrollToBlock(targetBlockId);
+            await _interactionService.RequestHighlightBlock(targetBlockId);
+        }
     }
 
     public async Task ProcessFullPageAsync(string fullContent, string tenantId = "global")

src/NexusReader.UI.Shared/Services/ReaderNavigationService.cs

@@ -62,6 +62,12 @@ public class ReaderNavigationService : IReaderNavigationService
         _navigationManager.NavigateTo($"/reader/{bookId}");
     }
 
+    public void SetBook(Guid bookId, int chapterIndex = 0)
+    {
+        CurrentEbookId = bookId;
+        CurrentChapterIndex = chapterIndex;
+    }
+
     private async Task NotifyNavigationChangedAsync()
     {
         var handlers = OnNavigationChanged?.GetInvocationList();