Issue 4535: Draggable cursor shows text caret

Goal

Fix issue #4535 by stopping the drag handle path from creating a Slate caret / DOM point inside the draggable UI, then verify with targeted tests and package checks.

Tracker Context

• Source: GitHub issue #4535
• Title: Draggable cursor has "input cusor"
• Task type: bug
• Browser surface: yes
• Reported browser: Firefox
• Reported error: Cannot resolve a Slate point from DOM point: [object HTMLDivElement],1

Relevant Instructions

• Use learnings first for non-trivial bug work.
• Use TDD when the seam is sane.
• Verify in the same turn before closeout.

Findings

• Existing DnD learning says fix the package seam when the drag layer owns the bug, not each caller.
• Current repro path likely runs through the official BlockDraggable registry UI, not a random app caller.
• The failing DOM point is an HTMLDivElement, which matches the nested drag-handle wrapper in apps/www/src/registry/ui/block-draggable.tsx.
• The durable seam is the registry draggable handle, not useDraggable: the package hook owns DnD refs, but the component owns the actual interactive DOM that Firefox can place a caret into.

Plan

1. Inspect the drag-handle chain and current regression coverage.
2. Reproduce at the smallest seam with a targeted test.
3. Implement the fix at the highest-leverage ownership boundary.
4. Run targeted tests, build/typecheck, lint, and browser verification if feasible.

Progress

• Fetched issue #4535 and comments.
• Read repo instructions plus relevant skills.
• Read DnD learning about package-level ownership.
• Inspected useDraggable, useDndNode, and the BlockDraggable registry component.
• Added an app-level regression spec that failed on the old nested button shape.
• Reworked BlockDraggable to render a single non-editable button as the tooltip trigger and cancel browser default on mousedown.
• Re-ran the new regression test after the fix.

Verification Target

• Targeted regression test for the drag-handle caret path.
• pnpm -C apps/www typecheck
• pnpm lint:fix
• Browser verification on the draggable demo if feasible.

Verification Results

• bun test apps/www/src/registry/ui/block-draggable.spec.tsx
• pnpm -C apps/www typecheck
• pnpm lint:fix
• Browser verification blocked: local http://localhost:3002/docs/dnd hit an unrelated docs-app build error in is-hotkey before the DnD example rendered.