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/pages/admin/UserDetailPage.tsx

@@ -3,6 +3,8 @@ import { useParams, useNavigate } from 'react-router-dom'
 import { ArrowLeft, Shield, Crown, UserCheck, UserX, Clock, Ticket, KeyRound, Copy, Check, Archive, ArchiveRestore, Trash2 } from 'lucide-react'
 import { StatusBadge } from '@/components/admin'
 import { Modal } from '@/components/common/Modal'
+import { Spinner } from '@/components/common/Spinner'
+import { EmptyState } from '@/components/common/EmptyState'
 import { adminApi } from '@/api/admin'
 import { toast } from '@/lib/toast'
 import { cn } from '@/lib/utils'
@@ -177,14 +179,25 @@ export function UserDetailPage() {
   if (loading) {
     return (
       <div className="flex items-center justify-center py-20">
-        <div className="h-8 w-8 animate-spin rounded-full border-2 border-border border-t-foreground" />
+        <Spinner className="border-t-foreground" />
       </div>
     )
   }
 
   if (!user) {
     return (
-      <div className="py-20 text-center text-muted-foreground">User not found</div>
+      <EmptyState
+        title="User not found"
+        description="This user may have been removed or is unavailable."
+        action={(
+          <button
+            onClick={() => navigate('/admin/users')}
+            className="rounded-md border border-border px-4 py-2 text-sm text-muted-foreground hover:bg-accent hover:text-foreground"
+          >
+            Back to Users
+          </button>
+        )}
+      />
     )
   }
 
@@ -202,7 +215,7 @@ export function UserDetailPage() {
           <ArrowLeft className="h-4 w-4" />
         </button>
         <div className="flex-1">
-          <h1 className="text-xl font-semibold text-foreground">
+          <h1 className="text-xl font-heading font-semibold text-foreground">
             {user.full_name || user.email}
           </h1>
           <p className="text-sm text-muted-foreground">{user.email}</p>