- plugin.json type field: 'content' (sidebar) or 'system' (API only)
- Content-edit: label 'Plugins', hide section if layout has no sidebar
- Content-edit: disable checkboxes when no sidebar layout selected
- Content-edit: hide up arrow on first item, down arrow on last item
- PluginManager: isPluginViewable() checks type=system -> not viewable
- Admin plugins page: show Content/Systeem type badge
- HTMLBlock: add plugin.json with type=content
- Navigation: add type=content to config
