Multi-selection properties panel with tests
Show position, appearance, fill, stroke, and effects sections
when multiple nodes are selected. Shared values display normally,
differing values show 'Mixed'. Editing applies to all selected.
- Shared composable (use-multi-props) for isMulti, active, activeNode
- ScrubInput supports MIXED symbol as model-value
- Per-node previous values for correct undo on multi-edit
- Fix flip transform encoding (scale first column only)
- Fix npm publish auth for MCP package
- 22 new tests: flip, alignment, multi-node merging, kiwi roundtrip