#3463300: Implement the concept of sections within the client

Remaining work:

• Make end-to-end tests pass in add-section.cy.js: issues with testing code accessing elements inside iframe
• Review and potentially revisit how preview thumbnails for the listed components and sections in ListItem.tsx are rendered. There is a failing test currently in xb-general.cy.js that seems to be showing a real bug. Plus, this might be a good opportunity to figure out how those preview thumbnails will play nicely with overflow: scroll added in #3463798: Unable to scroll when dragging components + unable to scroll component list.
• Add comments to explain layoutModelSlice.ts::replaceUUIDsAndUpdateModel() and layoutModelSlice.ts::insertMultipleNodes()
• Implement error handling in layoutModelSlice.ts::insertMultipleNodes()
• Verify the use of data-xb-component-id vs. data-xb-type in \Drupal\experience_builder\Controller\SdcController::wrapComponentsForPreview and in the client app
• Implement error handling in ComponentList.tsx
• Implement error handling in SectionList.tsx
• Tidy up code in Viewport.tsx
• Make mock data in ui/src/services/sections.ts a bit nicer — Let's approach this more holistically later
• Add error handling around <ComponentList /> and <SectionList />
• Nice to have: Refactor layoutModelSlice.ts::replaceUUIDsAndUpdateModel() and layoutModelSlice.ts::insertMultipleNodes(), make them unit testable, then write unit tests