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/admin/PageHeader.tsx

@@ -1,25 +1,2 @@
-import type { ReactNode } from 'react'
-import { cn } from '@/lib/utils'
-
-interface PageHeaderProps {
-  title: string
-  description?: string
-  action?: ReactNode
-  className?: string
-}
-
-export function PageHeader({ title, description, action, className }: PageHeaderProps) {
-  return (
-    <div className={cn('flex items-start justify-between gap-4', className)}>
-      <div>
-        <h1 className="text-2xl font-bold font-heading text-foreground">{title}</h1>
-        {description && (
-          <p className="mt-1 text-sm text-muted-foreground">{description}</p>
-        )}
-      </div>
-      {action && <div className="flex-shrink-0">{action}</div>}
-    </div>
-  )
-}
-
-export default PageHeader
+export { PageHeader } from '@/components/common/PageHeader'
+export { PageHeader as default } from '@/components/common/PageHeader'