fix: constrain tree editor page height to prevent panel overflow

Add overflow-hidden to TreeEditorPage root and NodeEditorPanel container
so the flex height chain is properly constrained by the CSS Grid cell,
preventing the node editor sidebar from growing beyond the viewport.

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

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

@@ -162,7 +162,7 @@ export function NodeEditorPanel({ nodeId, onClose, onSelectType }: NodeEditorPan
   const isRoot = treeStructure?.id === nodeId
 
   return (
-    <div ref={panelRef} className="flex h-full min-h-0 w-[400px] shrink-0 flex-col border-l border-border bg-card">
+    <div ref={panelRef} className="flex h-full min-h-0 w-[400px] shrink-0 flex-col border-l border-border bg-card overflow-hidden">
       {/* Header */}
       <div className="flex items-center gap-2 border-b border-border px-4 py-3 shrink-0">
         <span className={cn('flex h-5 w-5 shrink-0 items-center justify-center rounded', config.badgeClass)}>

frontend/src/pages/TreeEditorPage.tsx

@@ -407,7 +407,7 @@ export function TreeEditorPage() {
   }
 
   return (
-    <div className="flex h-full flex-col">
+    <div className="flex h-full flex-col overflow-hidden">
 
       {/* Draft Restore Prompt */}
       {showDraftPrompt && (