fix: KB Accelerator tree builder, approve all, canvas delete button

- Fix _build_troubleshooting_tree() to handle deep nesting, warning nodes,
  and DAG deduplication (placed set prevents duplicate IDs)
- Fix step_sync VARCHAR(255) overflow on publish by truncating title
- Add "Approve All" button to KB review screen
- Add delete button (hover-reveal) to flow canvas nodes

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

frontend/src/components/kb-accelerator/ReviewScreen.tsx

@@ -1,5 +1,5 @@
 import { useState } from 'react'
-import { CheckCircle2, AlertTriangle, BarChart3 } from 'lucide-react'
+import { CheckCircle2, AlertTriangle, BarChart3, CheckCheck } from 'lucide-react'
 import { cn } from '@/lib/utils'
 import { NodeCard } from './NodeCard'
 import { SourcePanel } from './SourcePanel'
@@ -8,12 +8,13 @@ import type { KBImport, KBNodeEditRequest, KBCommitRequest } from '@/types/kbAcc
 interface ReviewScreenProps {
   kbImport: KBImport
   onEditNode: (nodeId: string, data: KBNodeEditRequest) => Promise<void>
+  onApproveAll: () => Promise<void>
   onCommit: (data?: KBCommitRequest) => Promise<void>
   onDiscard: () => Promise<void>
   loading: boolean
 }
 
-export function ReviewScreen({ kbImport, onEditNode, onCommit, onDiscard, loading }: ReviewScreenProps) {
+export function ReviewScreen({ kbImport, onEditNode, onApproveAll, onCommit, onDiscard, loading }: ReviewScreenProps) {
   const [highlightExcerpt, setHighlightExcerpt] = useState<string | null>(null)
 
   const nodes = kbImport.nodes
@@ -57,6 +58,21 @@ export function ReviewScreen({ kbImport, onEditNode, onCommit, onDiscard, loadin
               </span>
             </div>
           )}
+          {approvedCount < nodes.length && (
+            <button
+              onClick={onApproveAll}
+              disabled={loading}
+              className={cn(
+                'flex items-center gap-1.5 px-3 py-1.5 rounded-[8px] text-xs font-medium transition-colors',
+                'bg-emerald-400/10 border border-emerald-400/20 text-emerald-400',
+                'hover:bg-emerald-400/20 hover:border-emerald-400/30',
+                'disabled:opacity-50 disabled:cursor-not-allowed'
+              )}
+            >
+              <CheckCheck size={14} />
+              Approve All
+            </button>
+          )}
         </div>
       </div>
 
@@ -98,7 +114,7 @@ export function ReviewScreen({ kbImport, onEditNode, onCommit, onDiscard, loadin
       </div>
 
       {/* Actions */}
-      <div className="flex items-center justify-between gap-3">
+      <div className="shrink-0 flex items-center justify-between gap-3 pb-1">
         <button
           onClick={onDiscard}
           disabled={loading}