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/Services/PromptRegistry.cs

@@ -6,6 +6,7 @@ public static class PromptRegistry
         "You are an expert educator. Analyze the provided text to extract key concepts, generate relevant quizzes, and construct a knowledge graph. " +
         "CRITICAL: Restrict 'concept.label' to a maximum of 3 words (e.g., 'Dependency Injection' instead of full sentences). " +
         "CRITICAL: Extract a MAXIMUM of 15 key concepts/plot points from the text. " +
+        "CRITICAL: Code blocks (e.g., markdown code snippets) must be excluded from the relationship graph, or summarized as a single node (e.g., 'Code Example'). Do NOT create nodes for variables, functions, namespaces, or individual lines of code. " +
         "CRITICAL: Return ONLY a minified JSON object. Do NOT include markdown formatting like ```json or ```. Do NOT include explanations. " +
         "Schema: { " +
         "\"concepts\": [ { \"title\": \"string\", \"description\": \"string\" } ], " +
@@ -14,13 +15,14 @@ public static class PromptRegistry
         "}.";
 
     public const string GraphExtractionPrompt = 
-        "You are an expert at information architecture. Extract key concepts and their relationships from the text to build a knowledge graph. " +
-        "CRITICAL: Restrict 'label' to a maximum of 3 words. " +
-        "CRITICAL: Extract a MAXIMUM of 15 key concepts/plot points and their relationships. " +
-        "CRITICAL: Each paragraph in the user text starts with [ID: some-id]. Use these IDs ONLY for nodes representing the blocks. " +
-        "CRITICAL: All other extracted 'concept' nodes MUST have unique, slug-style IDs based on their labels (e.g., 'dependency-injection'). " +
-        "Include a 'current' node representing the block content itself if applicable. " +
-        "CRITICAL: Limit the result to a MAXIMUM of 15 most relevant connections. " +
+        "You are an expert at information architecture. Extract key concepts and paragraph mappings from the text to build a unified knowledge graph. " +
+        "The input text consists of several paragraphs, each starting with its unique block ID in the format '[ID: seg-X]'. " +
+        "Extract two types of nodes: " +
+        "1. Concept Nodes (group: 'concept'): Extract the main technical concepts discussed (e.g., ID: 'dependency-injection', label: 'Dependency Injection'). Max 10 concepts. Labels must be at most 3 words. " +
+        "2. Block Nodes (group: 'current'): For each paragraph in the input, create a node representing that paragraph where 'id' is the exact block ID (e.g., 'seg-1'), and 'label' is a brief summary of that paragraph's content (max 3 words). " +
+        "CRITICAL: If a paragraph is a code block, represent it as a single block node with label 'Code Example' (group: 'current'). Do NOT extract low-level code elements (like variables, classes, methods, or namespaces) as separate concept nodes. " +
+        "CRITICAL: Connect related concept nodes together, and connect each concept node to the block nodes ('seg-X') where it is discussed. " +
+        "Limit connections to a MAXIMUM of 15 most relevant links. " +
         "Return ONLY minified JSON. Schema: { \"graph\": { \"nodes\": [ { \"id\": \"string\", \"label\": \"string\", \"group\": \"concept|current\" } ], \"links\": [ { \"source\": \"string\", \"target\": \"string\", \"value\": 1 } ] } }";
 
 
@@ -32,6 +34,7 @@ public static class PromptRegistry
         "You are an expert at Knowledge Engineering. Segment the provided text into discrete Knowledge Units. " +
         "Identify 'units' (sections, tables, definitions, rules) and 'links' (how they relate). " +
         "CRITICAL: Units must be granular. " +
+        "CRITICAL: Code blocks must be summarized under the parent unit or represented as a single 'Code Example' unit. Do NOT segment code blocks into granular low-level code details (e.g., classes, variables, parameters). " +
         "Schema: { " +
         "\"units\": [ { \"id\": \"string\", \"type\": \"Section|Table|Definition|Rule\", \"content\": \"string\", \"metadata\": { \"page\": 0 } } ], " +
         "\"links\": [ { \"source\": \"string\", \"target\": \"string\", \"relation\": \"Next|Defines|Contains|References\" } ] " +