Render app builder in arrange mode (#9260)

## Summary

Adds app builder in arrange/preview mode with re-orderable widgets,
maintaining size (as much as possible) between the select + preview
steps

## Changes

- **What**: 
- Extract sidebar size constants for sharing between canvas splitter +
app mode
- Add widget list using DraggableList and render inert WidgetItems

## Screenshots (if applicable)

<img width="1391" height="923" alt="image"
src="https://github.com/user-attachments/assets/3e17eafe-db1e-40a3-83b5-15a7d0672909"
/>

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9260-Render-app-builder-in-arrange-mode-3136d73d365081ef875acab683d01d9e)
by [Unito](https://www.unito.io)

src/composables/useAppMode.ts

@@ -16,8 +16,10 @@ export function useAppMode() {
   )
 
   const isBuilderMode = computed(
-    () => mode.value === 'builder:select' || mode.value === 'builder:arrange'
+    () => isSelectMode.value || isArrangeMode.value
   )
+  const isSelectMode = computed(() => mode.value === 'builder:select')
+  const isArrangeMode = computed(() => mode.value === 'builder:arrange')
   const isAppMode = computed(
     () => mode.value === 'app' || mode.value === 'builder:arrange'
   )
@@ -36,6 +38,8 @@ export function useAppMode() {
     mode,
     enableAppBuilder,
     isBuilderMode,
+    isSelectMode,
+    isArrangeMode,
     isAppMode,
     isGraphMode,
     setMode