fix: make canvas card expanded area scrollable with sticky header

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

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

@@ -166,7 +166,8 @@ export function TreeCanvasNode({
         className={cn(
           'flex items-center gap-2 px-3 py-2.5',
           !isExpanded && 'cursor-pointer hover:bg-accent/50 rounded-t-xl',
-          !isExpanded && 'rounded-xl'
+          !isExpanded && 'rounded-xl',
+          isExpanded && 'sticky top-0 z-10 bg-card rounded-t-xl'
         )}
         onClick={!isExpanded ? handleCardClick : undefined}
       >
@@ -322,7 +323,7 @@ export function TreeCanvasNode({
 
       {/* Expanded editing area */}
       {isExpanded && (
-        <div className="border-t border-border px-3 pb-3 pt-3">
+        <div className="border-t border-border px-3 pb-3 pt-3 max-h-[70vh] overflow-y-auto">
           {/* Validation errors */}
           {(hasError || hasWarning) && (
             <div className="mb-3 space-y-1">