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/AccountSettingsPage.tsx

@@ -138,12 +138,10 @@ export function AccountSettingsPage() {
 
   if (error) {
     return (
-      <div className="container mx-auto px-4 py-6 sm:px-6 sm:py-8">
-        <div className="rounded-md border border-red-400/20 bg-red-400/10 p-4 text-red-400">
-          <div className="flex items-center gap-2">
-            <AlertCircle className="h-5 w-5" />
-            {error}
-          </div>
+      <div className="rounded-md border border-red-400/20 bg-red-400/10 p-4 text-red-400">
+        <div className="flex items-center gap-2">
+          <AlertCircle className="h-5 w-5" />
+          {error}
         </div>
       </div>
     )
@@ -152,7 +150,7 @@ export function AccountSettingsPage() {
   const sub = subscription?.subscription
 
   return (
-    <div className="container mx-auto px-4 py-6 sm:px-6 sm:py-8">
+    <div>
       <div className="mb-8">
         <div className="flex items-center gap-3">
           <Building2 className="h-8 w-8 text-muted-foreground" />