feat: dual-stream mindmap output with JSON mode and mindmap-as-context

[web4zn]

• Single API call produces both chat answer and full updated mindmap tree
• JSON mode (response_format: json_object) for providers that support it
• Fallback marker mode for other providers
• Mindmap tree replaces raw conversation history as LLM context (60-80% token savings)
• Removed corpus, batch generation, knowledge-applier, and all intermediate types
• Simplified MindMap data model (monitoredConversationIds only)
• Conversation-mindmap association via new dialog and panel
• Edited node preservation via findEditedNodes/mergeEditedNodes
• Increased dagre layout spacing for dense trees

Summary

Brief description of the change and why it's needed.

Related Issue

Fixes # (issue number)

Type of Change

• Bug fix
• New feature
• Refactoring
• Documentation
• Other (please describe):

Checklist

• I have tested my changes locally (npm test)
• TypeScript compiles clean (npx tsc --noEmit)
• I have updated documentation if needed
• This PR follows existing code patterns in the project