Authoring: Main scene, HUD panel, move FurShell material to Assets/Materials

- Main.unity + PaintHudPanel.prefab: fur/paint HUD authoring updates.
- Rename FurShell_Test.mat -> Materials/FurShell.mat (same GUID).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Materials/FurShell.mat.meta

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