feat: add AI Chat block with contextual prompting and persistence

[flooryyyy]

Motivation

• Provide an on-canvas AI assistant block that accepts a user prompt and can use selected blocks as context to produce helpful assistant responses.
• Allow optional expansion of context to directly linked nodes (depth = 1) so users can surface nearby related content automatically.
• Persist assistant messages and chat configuration on the block so conversations survive reloads and can be reviewed later.

Description

• Add a new chat block type and UI AIChatBlock component with prompt input, checkboxes for Include selected blocks and Expand linked nodes (depth 1), assistant response display, and local styling (ai-chat-block.css).
• Wire the block into the app: include chat in block type unions and the Add Block modal, register AIChatBlock in ProjectCanvas and PublicProjectCanvas, and pass allBlocks/allLinks into node data so the block can assemble selected context and linked-node expansion.
• Persist chat state and assistant replies in block metadata.messages and initialize chat metadata defaults (includeSelectedContext: true, expandLinkedNodes: false) when creating new chat blocks.
• Add backend endpoint POST /api/ai/chat that accepts prompt + assembled context, calls the OpenAI Responses API when OPENAI_API_KEY is configured, and returns a harmless fallback message when the key is not set.
• Update type/schema layers and DB support: extend graph validation and blocks DB type union to include chat, and add migration 34AddChatBlockType.ts to allow chat as a valid blockType.
• Add i18n keys for chat UI in en.json, fr.json, and it.json.

Testing

• Ran static/type checks with npm run -s check which succeeded.
• Ran the targeted unit tests npm run -s test -- src/app/lib/graph.test.ts which passed.
• The full test suite (vitest run) failed in this environment with an existing local sqlite migration/test DB error (SqliteError: no such table: main.blocks_old), so the broader CI-style test run did not complete here due to that database state issue.

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Codex Task