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/components/rightSidePanel/parameters/WidgetItem.vue

@@ -34,6 +34,7 @@ const {
   node,
   isDraggable = false,
   hiddenFavoriteIndicator = false,
+  hiddenWidgetActions = false,
   showNodeName = false,
   parents = [],
   isShownOnParents = false
@@ -42,6 +43,7 @@ const {
   node: LGraphNode
   isDraggable?: boolean
   hiddenFavoriteIndicator?: boolean
+  hiddenWidgetActions?: boolean
   showNodeName?: boolean
   parents?: SubgraphNode[]
   isShownOnParents?: boolean
@@ -170,7 +172,10 @@ const displayLabel = customRef((track, trigger) => {
       >
         {{ sourceNodeName }}
       </span>
-      <div class="flex items-center gap-1 shrink-0 pointer-events-auto">
+      <div
+        v-if="!hiddenWidgetActions"
+        class="flex items-center gap-1 shrink-0 pointer-events-auto"
+      >
         <WidgetActions
           v-model:label="displayLabel"
           :widget="widget"