[6027] Improve visual feedback for diagram DnD

[pcdavid]

Improve visual feedback when dragging a node to show which targets are compatible/valid/invalid for a drop

Bug: #6027

Capture.video.du.2026-01-09.09-29-07.mp4

Studio and instance to reproduce/test:

• DnD Studio.zip
• DnD Instance.zip

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[sbegaudeau]

This change seems to create an issue with the Playwright test for the drag and drop

[sbegaudeau]

I'll merge it after the homologation phase