ENG-1691: Open ModifyNodeModal for convert-into flow with existing node support

[trangdoan982]

https://www.loom.com/share/7f235985b2d8403c9d4e3164578192f5

Summary

• Opens ModifyNodeModal pre-filled with shape content when converting a text/image shape via the canvas "Convert to" context menu
• When the user selects an existing discourse node from the modal's search results, that node is inserted as a new canvas shape directly — no new file is created
• For image shapes, image content is only embedded in newly created nodes; existing nodes are not modified (but the canvas shape still shows the image as a preview)
• Adds docs for the convert-to-shape flow in the Obsidian canvas feature page

Test plan

• Right-click a text shape → Convert to → select a node type → modal opens pre-filled with text
  • Type a new title → Confirm → new discourse node file is created and shape is replaced
  • Search for existing node → select it → Confirm → existing node is inserted as a shape, no new file created
• Right-click an image shape → Convert to → select a node type → modal opens with empty title
  • Enter a new title → Confirm → new file created with image embedded, shape replaced
  • Select an existing node → Confirm → existing node inserted as shape, no image embedded into the file
• Cancel the modal → original shape remains unchanged

🤖 Generated with Claude Code

[linear-code]

ENG-1691 Enable open ModifyNodeModal for “Convert into” flow for texts (from canvas, right click and file explorer)

[supabase]

This pull request has been ignored for the connected project zytfjzqyijgagqxrzbmz because there are no changes detected in packages/database/supabase directory. You can change this behaviour in Project Integrations Settings ↗︎.

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