v0.4.0: Multi-plate grid rendering & project normalization

Features

• Multi-plate grid rendering: All build plates now display simultaneously in a 2-column grid layout, matching desktop OrcaSlicer
• Automatic plate normalization: 3MF projects are loaded with correct plate offsets, accounting for the file's embedded bed size
• Full multi-plate support: Load, view, and slice on any plate; plate switching preserves all edits

Fixes

• Fixed offset calculation for multi-plate 3MF projects created on different bed sizes (e.g., 218×218 vs 300×300)
• Inactive plates render dimmed for visual distinction while maintaining full visibility of the workspace

Details

• Plates are laid out in a 2-column grid with 1.2× bed-size stride (matching desktop compute_colum_count and plate_stride_x/y logic)
• Object positions are normalized back to bed-local coordinates using the 3MF's embedded printable_area
• Active plate receives full rendering (colors, selection, gizmos); inactive plates are display-only dimmed