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/ReaderCanvas.razor.css

@@ -156,4 +156,106 @@
     0% { transform: scale(1); opacity: 1; }
     50% { transform: scale(1.1); opacity: 0.8; }
     100% { transform: scale(1); opacity: 1; }
+}
+
+/* Chapter Loading Overlay and Spinners */
+.loading-state.full-page {
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    justify-content: center;
+    height: 100%;
+    min-height: 400px;
+    gap: 1.5rem;
+    animation: fadeIn 0.4s ease-out;
+}
+
+.spinner-glow {
+    width: 60px;
+    height: 60px;
+    border: 3px solid rgba(0, 255, 153, 0.1);
+    border-radius: 50%;
+    border-top-color: var(--nexus-neon, #00ff99);
+    animation: spin 1s cubic-bezier(0.55, 0.055, 0.675, 0.19) infinite;
+    box-shadow: 0 0 15px rgba(0, 255, 153, 0.2);
+}
+
+.spinner-glow.small {
+    width: 36px;
+    height: 36px;
+    border-width: 2px;
+}
+
+.loading-text {
+    color: rgba(255, 255, 255, 0.7);
+    font-size: 1.1rem;
+    letter-spacing: 0.5px;
+    font-weight: 500;
+}
+
+.theme-light .loading-text {
+    color: rgba(0, 0, 0, 0.7);
+}
+
+.chapter-loading-overlay {
+    position: absolute;
+    top: 0;
+    left: 0;
+    width: 100%;
+    height: 100%;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    z-index: 10;
+    background-color: rgba(15, 23, 42, 0.15);
+    backdrop-filter: blur(2px);
+    animation: fadeIn 0.3s ease-out;
+}
+
+.theme-light .chapter-loading-overlay {
+    background-color: rgba(255, 255, 255, 0.2);
+}
+
+.loader-card {
+    display: flex;
+    align-items: center;
+    gap: 1.25rem;
+    padding: 1.25rem 2rem;
+    border-radius: 40px;
+    box-shadow: 0 10px 30px rgba(0, 0, 0, 0.25), 0 0 1px rgba(255, 255, 255, 0.1) inset;
+    animation: scaleIn 0.3s cubic-bezier(0.34, 1.56, 0.64, 1);
+}
+
+.loader-text {
+    font-family: var(--nexus-font-sans, 'Inter', sans-serif);
+    font-weight: 500;
+    color: #ffffff;
+    font-size: 0.95rem;
+    letter-spacing: 0.2px;
+}
+
+.theme-light .loader-text {
+    color: #0f172a;
+}
+
+.content-blurred {
+    filter: blur(3px);
+    opacity: 0.55;
+    transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
+    pointer-events: none;
+}
+
+@keyframes spin {
+    0% { transform: rotate(0deg); }
+    100% { transform: rotate(360deg); }
+}
+
+@keyframes scaleIn {
+    from { transform: scale(0.9) translateY(10px); opacity: 0; }
+    to { transform: scale(1) translateY(0); opacity: 1; }
+}
+
+@keyframes fadeIn {
+    from { opacity: 0; }
+    to { opacity: 1; }
 }