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/step-library/CustomStepModal.tsx

@@ -5,6 +5,7 @@ import { usePermissions } from '@/hooks/usePermissions'
 import { StepForm } from './StepForm'
 import { StepLibraryBrowser } from './StepLibraryBrowser'
 import type { Step, StepCreate } from '@/types/step'
+import { Spinner } from '@/components/common/Spinner'
 
 export interface CustomStepDraft {
   title: string
@@ -134,7 +135,7 @@ export function CustomStepModal({ isOpen, onClose, onInsertStep }: CustomStepMod
         {isSubmitting && (
           <div className="absolute inset-0 flex items-center justify-center bg-black/80 backdrop-blur-sm">
             <div className="flex flex-col items-center gap-3">
-              <div className="h-8 w-8 animate-spin rounded-full border-4 border-border border-t-foreground" />
+              <Spinner className="border-t-foreground" />
               <p className="text-sm text-muted-foreground">Creating step...</p>
             </div>
           </div>