fix: add answer type to all Record<NodeType> icon and color maps

Fixes NodeList, ContinuationModal, NodePicker, and TreePreviewNode.

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

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

@@ -13,14 +13,16 @@ const CREATE_SOLUTION = `${CREATE_PREFIX}solution__`
 const NODE_TYPE_SYMBOLS: Record<NodeType, string> = {
   decision: '\u24D8',  // Information/question symbol
   action: '\u26A1',    // Lightning bolt for action
-  solution: '\u2713'   // Checkmark for solution
+  solution: '\u2713',  // Checkmark for solution
+  answer: '\u25CC'     // Dashed circle for placeholder
 }
 
 // Node type labels for UI
 const NODE_TYPE_LABELS: Record<NodeType, string> = {
   decision: 'Decision',
   action: 'Action',
-  solution: 'Solution'
+  solution: 'Solution',
+  answer: 'Answer'
 }
 
 interface NodePickerProps {