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/layout/ProtectedRoute.tsx

@@ -1,6 +1,7 @@
 import { Navigate, useLocation } from 'react-router-dom'
 import { useAuthStore } from '@/store/authStore'
 import { usePermissions, type EffectiveRole } from '@/hooks/usePermissions'
+import { Spinner } from '@/components/common/Spinner'
 
 interface ProtectedRouteProps {
   requiredRole?: EffectiveRole
@@ -15,7 +16,7 @@ export function ProtectedRoute({ requiredRole, children }: ProtectedRouteProps)
   if (isLoading) {
     return (
       <div className="flex h-screen items-center justify-center">
-        <div className="h-8 w-8 animate-spin rounded-full border-4 border-border border-t-foreground" />
+        <Spinner className="border-t-foreground" />
       </div>
     )
   }