Pinia store + basic builder canvas

[tbssnch]

With the data layer proven and the drag-and-drop interaction validated, wire them together into the actual builder UI. This sets up the Pinia store as the single source of truth and renders the form tree on a canvas with working drag-and-drop.

Pinia store

• Holds the XLSFormDocument
• Exposes actions:
  • addNode(parentId, index, type)
  • moveNode(nodeId, newParentId, newIndex)
  • removeNode(nodeId)
  • updateNode(nodeId, patch)
  • addChoiceList(listName) / addChoice(listName, choice) / removeChoice(listName, index)
• Undo/redo via structuredClone()
• No component mutates the tree directly. All mutations go through store actions

Builder canvas

• Renders the survey tree recursively
• Each node renders as a card/row showing type icon + label + name
• Groups and repeats render as containers with their children indented inside
• Drag-and-drop wired up via vue-draggable-plus (using patterns validated in Nested drag-and-drop prototype #24)
• Clicking a node selects it (selection state in the store for the property panel in a later issue)

Question palette sidebar

• List of available question types (text, integer, select_one, note, group, repeat, etc)
• Draggable items. Node is created on drop not on drag start

Export button

• Calls the export pipeline from Tree utilities + XLSForm export pipeline #23
• Downloads the .xlsx file via browser download