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/TreeNavigationPage.tsx

@@ -795,7 +795,7 @@ export function TreeNavigationPage() {
                   {index < 9 && (
                     selectingOption === option.id ? (
                       <span className="flex h-6 w-6 shrink-0 items-center justify-center">
-                        <span className="h-4 w-4 animate-spin rounded-full border-2 border-border border-t-foreground" />
+                        <Spinner size="sm" className="h-4 w-4 border-t-foreground" />
                       </span>
                     ) : (
                       <span className="flex h-6 w-6 shrink-0 items-center justify-center rounded bg-accent text-xs font-medium text-muted-foreground">