[Feature] Add mind map node editing (add, rename, delete)

[d-oit]

Description

The mind map view (MindMapView.tsx) displays entities and links as a tree structure using Mind Elixir, but users cannot edit the mind map — no adding, renaming, or deleting nodes. The mind map is currently read-only.

Current State

• ✅ Mind Elixir integration with tree builder
• ✅ Root entity selector, depth control, relation filter
• ❌ No node editing (add/rename/delete)
• ❌ No persistence of mind map layout changes
• ❌ No export of mind map as image

Proposed Implementation

1. Enable Mind Elixir's built-in editing features (it supports drag, add, delete)
2. Add toolbar buttons for add child, add sibling, rename, delete
3. Sync mind map edits back to the database (create/update entities)
4. Add "Export as PNG" button using html2canvas or similar
5. Persist custom layout positions in localStorage

Acceptance Criteria

• Users can add child and sibling nodes
• Users can rename nodes
• Users can delete nodes (with confirmation)
• Edits sync to database
• Export as PNG available
• Layout positions persist in localStorage

[github-actions]

👋 This looks like a duplicate of #181.

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