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.Infrastructure/DependencyInjection.cs

@@ -63,7 +63,12 @@ public static class DependencyInjection
             builder.AddRetry(new RetryStrategyOptions
             {
                 ShouldHandle = new PredicateBuilder().Handle<Exception>(ex =>
-                    ex.Message.Contains("429") || ex.Message.Contains("Too Many Requests") || ex.Message.Contains("quota")),
+                    ex.Message.Contains("429") || 
+                    ex.Message.Contains("Too Many Requests") || 
+                    ex.Message.Contains("quota") ||
+                    ex.Message.Contains("503") ||
+                    ex.Message.Contains("ServiceUnavailable") ||
+                    ex.Message.Contains("demand")),
                 BackoffType = DelayBackoffType.Exponential,
                 UseJitter = true,
                 MaxRetryAttempts = aiSettings.RetryAttempts,
@@ -80,7 +85,7 @@ public static class DependencyInjection
         services.AddEmbeddingGenerator(new GeminiEmbeddingGenerator(new GeminiClientOptions
         {
             ApiKey = aiSettings.ApiKey,
-            ModelId = aiSettings.EmbeddingModel ?? "text-embedding-004"
+            ModelId = aiSettings.EmbeddingModel ?? "gemini-embedding-001"
         }));
 
         // Application-layer service implementations