refactor: Update forms for inline safety, add MetadataSidePanel, update layout

- NodeFormDecision: option reorder via onUpdate (no premature store writes)
- NodePicker: add allowCreate prop (default true) to hide Create New options
  during inline canvas editing, preventing side-effect node creation
- MetadataSidePanel: 320px right slide-in overlay wrapping TreeMetadataForm,
  closes on backdrop click, close button, and Escape key
- TreeEditorLayout: Flow mode now renders full-width TreeCanvas + MetadataSidePanel
  overlay; Code mode unchanged (Monaco + preview split)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/tree-editor/NodeFormDecision.tsx

@@ -16,7 +16,7 @@ const indexToLetter = (index: number): string => {
 }
 
 export function NodeFormDecision({ node, onUpdate }: NodeFormDecisionProps) {
-  const { reorderOptions, validationErrors } = useTreeEditorStore()
+  const { validationErrors } = useTreeEditorStore()
   const isRootNode = node.id === 'root'
 
   const questionError = validationErrors.find(
@@ -51,7 +51,12 @@ export function NodeFormDecision({ node, onUpdate }: NodeFormDecisionProps) {
   }
 
   const handleReorderOptions = (fromIndex: number, toIndex: number) => {
-    reorderOptions(node.id, fromIndex, toIndex)
+    // Mutate local draft via onUpdate (backward-compatible: modal path relays to store,
+    // canvas path updates local draft without writing to store early)
+    const newOptions = [...(node.options || [])]
+    const [moved] = newOptions.splice(fromIndex, 1)
+    newOptions.splice(toIndex, 0, moved)
+    onUpdate({ options: newOptions })
   }
 
   return (