[ENG-1546] Relation creation via drag handles (Roam)

[trangdoan982]

Summary

• Adds drag handle dots on selected discourse nodes (4 edge midpoints) for creating relations by dragging to another node
• Shows a relation type dropdown at the midpoint between source/target nodes after a successful drag
• On type selection, creates the relation shape with correct type and bindings, and persists via handleCreateRelationsInRoam (supports both stored/reified relations and legacy triple-based approach)
• Adapted from the Obsidian implementation ([ENG-1547] Relation creation via drag handle (Obsidian) #909) to fit Roam's per-relation-type shape architecture — uses SVG overlay line during drag instead of temporary tldraw shapes

Test plan

• Select a single discourse node — 4 drag handle dots appear at edge midpoints
• Drag from a handle to another discourse node — SVG line follows cursor, turns blue on hover over valid target
• Release on a valid target — relation type dropdown appears at midpoint
• Select a relation type — arrow shape created with correct color/label, relation persisted (check stored relations page or triple blocks)
• Release on empty space — toast warning, no arrow created
• Release on a node with no valid relation types — toast warning, no arrow created
• Escape or click outside dropdown — dropdown dismissed, no arrow created
• Handles reappear when selecting a different discourse node after any flow

🤖 Generated with Claude Code

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[linear]

ENG-1546 Implement relation creation via drag handles in tldraw (Roam)

[supabase]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.