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/maintenance/BatchLaunchModal.tsx

@@ -4,6 +4,7 @@ import { cn } from '@/lib/utils'
 import { toast } from '@/lib/toast'
 import { targetListsApi, batchLaunchApi } from '@/api'
 import type { TargetList, TargetEntry } from '@/types'
+import { Spinner } from '@/components/common/Spinner'
 
 interface BatchLaunchModalProps {
   treeId: string
@@ -127,7 +128,7 @@ export function BatchLaunchModal({ treeId, treeName, onClose, onLaunched }: Batc
             <div className="space-y-2">
               {savedLists === null ? (
                 <div className="flex h-24 items-center justify-center">
-                  <div className="h-4 w-4 animate-spin rounded-full border-2 border-primary border-t-transparent" />
+                  <Spinner size="sm" className="border-primary border-t-transparent" />
                 </div>
               ) : savedLists.length === 0 ? (
                 <p className="text-[0.875rem] text-muted-foreground">