Add drag-and-drop layer reordering in Layers panel

[Patolord]

Summary

• Adds drag-and-drop reordering for floors in the Layers panel, with a visual insertion indicator while dragging.
• Introduces reorderLayer() to update floor order, shape/edge layer indices, active floor selection, and board rendering.
• Commits reorder on dragend using the last valid drop target from dragover, which keeps Chrome drag behavior reliable.

Closes #21

Test plan

• npm run build
• npx vitest run test/reorderLayer.test.ts
• Manual Chrome drag in the Layers panel: reorder floors and confirm canvas elevation updates immediately
• Verify drop indicator appears between rows while dragging
• Verify reorder is disabled when only one floor exists
• Verify dragging from layer controls (buttons, color picker) does not start a drag

Made with Cursor