[ENG-1547] Relation creation via drag handle (Obsidian)

[trangdoan982]

https://www.loom.com/share/ee0d603ff9db4025a1c8f857f2b01aec

Summary

• Clicking an existing persisted relation arrow now shows the relation type dropdown at the arrow midpoint
• Selecting a new type updates the arrow visuals (color, label) and persists the change to relations.json in-place (preserving id, created, author, etc.)
• Added updateRelationType() helper to relationsStore.ts
• Handles edge cases: same-type re-selection dismisses without changes, Escape/click-outside closes without modifying, dropdown doesn't re-open after creation or edit flows

Test plan

• Create a relation via drag handles — existing flow still works, dropdown closes after selection
• Select a new type — arrow updates (color, label) and relations.json type field updated in-place
• Escape/click outside — dropdown closes, arrow disappear
• Relation Overlay updated correctly after relation is created

🤖 Generated with Claude Code

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[linear]

ENG-1547 Implement relation creation via drag handles in tldraw (Obsidian)

[supabase]

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[mdroidian]

1. CI is failing.

2. This PR is ~50% over the line change limit. I understand this might be required (or ideal) in this case, that being said, would you mind creating a Loom video walking through this code and code choices?

The biggest concern I have doing a quick review was the code possibly not being as DRY as possible. I worry we might be duplicating work, specifically in the relation/arrow drag handling logic and potential relations checking.

[trangdoan982]

@mdroidian video explain file structure here: https://youtu.be/BBAKm1G-zXM