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/Pages/Account/Login.razor

@@ -117,6 +117,7 @@
                 "ProvisioningFailed" => "Wystąpił błąd podczas przygotowywania Twojego konta.",
                 "UserAlreadyExists" => "Użytkownik o tym adresie e-mail już istnieje. Zaloguj się tradycyjnie hasłem.",
                 "LockedOut" => "Twoje konto zostało zablokowane. Spróbuj ponownie później.",
+                "InvalidCredentials" => "Nieprawidłowy e-mail lub hasło.",
                 _ => "Wystąpił nieoczekiwany błąd podczas logowania."
             };
         }
@@ -132,8 +133,8 @@
             var result = await IdentityService.LoginAsync(_loginModel.Email, _loginModel.Password, _loginModel.RememberMe);
             if (result.IsSuccess) 
             {
-                // Trigger hidden form submission to perform cookie-based sign-in
-                await JS.InvokeVoidAsync("eval", "document.getElementById('nexusLoginForm').submit()");
+                // Trigger hidden form submission via robust JS helper to perform cookie-based sign-in
+                await JS.InvokeVoidAsync("nexusAuth.submitLoginForm", "nexusLoginForm", _loginModel.Email, _loginModel.Password, _loginModel.RememberMe);
             }
             else 
             {