Move reusable GUI primitives (PanelTile, EditablePanelTile, base Tile,
ElevatedPill, SingleLineTextEdit) into the shared dashgui package and
consume them via a file: dependency. EditablePanelTile now accepts an
onError prop so error notification stays app-side.
- Add setCategories API method for PUT /api/article/:sku/categories
- Create CategoryPickerDialog component for selecting categories
- Add CategoryPills component with add/remove functionality
- Category changes are now draft-based (like name editing)
- Save button commits both name and category changes together
- Delete icons hidden when only one category remains
- Plus button only visible in edit mode
Implements category assignment flow similar to name editing pattern.
