feat: add delete button with confirmation to AnswerStubCard

Adds an inline delete flow to answer stub placeholder cards:
- Trash icon button (top-right, subtle) visible in idle state
- Click reveals "Delete this stub?" confirmation with Delete/Cancel
- Confirmed delete calls onDelete(nodeId) wired to handleDelete in TreeCanvas

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

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

@@ -1,5 +1,5 @@
 import { useState } from 'react'
-import { HelpCircle, Zap, CheckCircle } from 'lucide-react'
+import { HelpCircle, Zap, CheckCircle, Trash2 } from 'lucide-react'
 import { cn } from '@/lib/utils'
 import type { TreeStructure } from '@/types'
 
@@ -7,28 +7,62 @@ interface AnswerStubCardProps {
   node: TreeStructure  // type === 'answer'
   fromOption?: string
   onSelectType: (nodeId: string, type: 'decision' | 'action' | 'solution') => void
+  onDelete: (nodeId: string) => void
 }
 
-export function AnswerStubCard({ node, fromOption, onSelectType }: AnswerStubCardProps) {
+export function AnswerStubCard({ node, fromOption, onSelectType, onDelete }: AnswerStubCardProps) {
   const [picking, setPicking] = useState(false)
+  const [confirming, setConfirming] = useState(false)
   const label = fromOption || node.title || 'Answer'
 
   return (
     <div
       className={cn(
-        'min-w-[180px] max-w-[280px] rounded-xl border-2 border-dashed border-border bg-card/50',
+        'relative min-w-[180px] max-w-[280px] rounded-xl border-2 border-dashed border-border bg-card/50',
         'transition-all duration-150',
-        !picking && 'cursor-pointer hover:border-primary/40 hover:bg-accent/30'
+        !picking && !confirming && 'cursor-pointer hover:border-primary/40 hover:bg-accent/30'
       )}
-      onClick={() => !picking && setPicking(true)}
+      onClick={() => !picking && !confirming && setPicking(true)}
     >
+      {/* Delete button — top-right corner */}
+      {!picking && !confirming && (
+        <button
+          type="button"
+          onClick={(e) => { e.stopPropagation(); setConfirming(true) }}
+          className="absolute top-1.5 right-1.5 rounded p-0.5 text-muted-foreground/40 hover:bg-red-500/10 hover:text-red-400 transition-colors"
+          title="Delete stub"
+        >
+          <Trash2 className="h-3 w-3" />
+        </button>
+      )}
+
       {/* Label */}
       <div className="px-3 pt-2.5 pb-1 text-sm font-heading font-medium text-foreground text-center">
         {label}
       </div>
 
-      {/* Prompt / type picker */}
-      {!picking ? (
+      {/* Confirm delete */}
+      {confirming ? (
+        <div className="px-2 pb-2.5 text-center space-y-1.5">
+          <p className="text-[10px] text-muted-foreground">Delete this stub?</p>
+          <div className="flex items-center justify-center gap-1.5">
+            <button
+              type="button"
+              onClick={(e) => { e.stopPropagation(); onDelete(node.id) }}
+              className="rounded-md px-2 py-1 text-[10px] font-label border border-red-500/30 bg-red-500/10 text-red-400 hover:bg-red-500/20"
+            >
+              Delete
+            </button>
+            <button
+              type="button"
+              onClick={(e) => { e.stopPropagation(); setConfirming(false) }}
+              className="rounded-md px-2 py-1 text-[10px] font-label border border-border text-muted-foreground hover:bg-accent"
+            >
+              Cancel
+            </button>
+          </div>
+        </div>
+      ) : !picking ? (
         <div className="pb-2.5 text-center text-[10px] text-muted-foreground font-label">
           + Choose Type
         </div>

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

@@ -503,6 +503,7 @@ export function TreeCanvas() {
               node={node}
               fromOption={optionLabel}
               onSelectType={handleSelectAnswerType}
+              onDelete={handleDelete}
             />
           ) : (
             <TreeCanvasNode