[codex] Honor min size during empty cell drag

[nexiumbiz-debug]

Summary

• size initial empty-cell items to at least the configured minItemCols/minItemRows
• keep empty-cell drag previews at the configured minimum size when dragging only horizontally or vertically
• preserve the starting cell as the anchor while expanding in the drag direction

Fixes #417.

Validation

• npm run test-lib -- --watch=false
• npm run build-lib
• npx eslint projects/angular-gridster2/src/lib/gridsterEmptyCell.ts projects/angular-gridster2/src/lib/tests/gridsterEmptyCell.spec.ts
• git diff --check