feat: AI flow builder, visibility model, dashboard tabs, fork UI (#88)

- AI flow builder: scaffold → branch detail → assemble → review flow
- Generate All one-click branch generation with stop/cancel
- Regenerate scaffold suggestions button
- 3-action review screen: Start Flow, Open in Editor, Build Another
- Fix Publish button gated behind !isDirty
- Fix visibility column enforcement in tree access filter
- Add ?visibility filter and author_name to GET /trees
- Dashboard tabbed flows: My Flows / My Team / Public / All
- Create button in My Flows tab, window focus reload (stale data fix)
- Fork UI with optional reason modal
- Fix account_id nullability in User type and schema
- Keep is_public and visibility in sync on updates

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

frontend/src/components/ai-builder/TreePreviewCard.tsx

@@ -1,13 +1,15 @@
-import { GitBranch, Layers, CheckCircle, ArrowRight, RotateCcw } from 'lucide-react'
+import { GitBranch, Layers, CheckCircle, ArrowRight, RotateCcw, Play } from 'lucide-react'
 import { useAIFlowBuilderStore } from '@/store/aiFlowBuilderStore'
 import { cn } from '@/lib/utils'
 
 interface TreePreviewCardProps {
   onOpenInEditor: () => void
+  onStartFlow: () => void
+  onBuildAnother: () => void
 }
 
-export function TreePreviewCard({ onOpenInEditor }: TreePreviewCardProps) {
-  const { assembledTree, reset, isLoading } = useAIFlowBuilderStore()
+export function TreePreviewCard({ onOpenInEditor, onStartFlow, onBuildAnother }: TreePreviewCardProps) {
+  const { assembledTree, isLoading } = useAIFlowBuilderStore()
 
   if (!assembledTree) return null
 
@@ -58,27 +60,38 @@ export function TreePreviewCard({ onOpenInEditor }: TreePreviewCardProps) {
       )}
 
       {/* Actions */}
-      <div className="flex gap-2">
+      <div className="flex flex-col gap-2">
         <button
           type="button"
-          onClick={onOpenInEditor}
+          onClick={onStartFlow}
           disabled={isLoading}
           className={cn(
-            'flex flex-1 items-center justify-center gap-2 rounded-lg bg-gradient-brand py-2.5 text-sm font-medium text-white shadow-lg shadow-primary/20',
-            'hover:opacity-90'
+            'flex w-full items-center justify-center gap-2 rounded-lg bg-gradient-brand py-2.5 text-sm font-medium text-white shadow-lg shadow-primary/20',
+            'hover:opacity-90 disabled:opacity-50'
           )}
         >
-          <ArrowRight className="h-4 w-4" />
-          Open in Editor
-        </button>
-        <button
-          type="button"
-          onClick={reset}
-          className="flex items-center gap-2 rounded-lg border border-border px-4 py-2.5 text-sm text-muted-foreground hover:bg-accent hover:text-foreground"
-        >
-          <RotateCcw className="h-4 w-4" />
-          Start Over
+          <Play className="h-4 w-4" />
+          Start Flow
         </button>
+        <div className="flex gap-2">
+          <button
+            type="button"
+            onClick={onOpenInEditor}
+            disabled={isLoading}
+            className="flex flex-1 items-center justify-center gap-2 rounded-lg border border-border py-2 text-sm text-muted-foreground hover:bg-accent hover:text-foreground disabled:opacity-50"
+          >
+            <ArrowRight className="h-4 w-4" />
+            Open in Editor
+          </button>
+          <button
+            type="button"
+            onClick={onBuildAnother}
+            className="flex items-center gap-2 rounded-lg border border-border px-4 py-2 text-sm text-muted-foreground hover:bg-accent hover:text-foreground"
+          >
+            <RotateCcw className="h-4 w-4" />
+            Build Another
+          </button>
+        </div>
       </div>
     </div>
   )