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/KnowledgeGraph.razor.css

@@ -10,6 +10,10 @@
     position: relative;
 }
 
+.knowledge-graph-container.loading > ::deep svg {
+    display: none !important;
+}
+
 .graph-controls {
     position: absolute;
     bottom: 1.5rem;
@@ -52,12 +56,27 @@
 }
 
 .loading-state {
+    position: absolute;
+    top: 50%;
+    left: 50%;
+    transform: translate(-50%, -50%);
     display: flex;
     flex-direction: column;
     align-items: center;
-    gap: 1.5rem;
+    justify-content: center;
+    gap: 1.25rem;
     color: #fff;
     text-align: center;
+    z-index: 20;
+    background: rgba(13, 13, 13, 0.8);
+    backdrop-filter: blur(8px);
+    padding: 2rem 1.5rem;
+    border-radius: 16px;
+    border: 1px solid rgba(255, 255, 255, 0.05);
+    box-shadow: 0 20px 40px rgba(0, 0, 0, 0.5);
+    width: 85%;
+    max-width: 280px;
+    box-sizing: border-box;
 }
 
 .preloader-robot {