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/session/ShareSessionModal.tsx

@@ -5,6 +5,7 @@ import { sessionsApi } from '@/api/sessions'
 import { buildSessionShareUrl, filterSharesForSession } from '@/lib/sessionShare'
 import { cn } from '@/lib/utils'
 import { toast } from '@/lib/toast'
+import { Spinner } from '@/components/common/Spinner'
 
 interface ShareSessionModalProps {
   sessionId: string
@@ -406,7 +407,7 @@ export function ShareSessionModal({ sessionId, sessionLabel, isOpen, onClose }:
           {/* Loading state */}
           {isLoadingShares && shares.length === 0 && (
             <div className="flex items-center justify-center py-4">
-              <div className="h-5 w-5 animate-spin rounded-full border-2 border-border border-t-foreground" />
+              <Spinner size="sm" className="h-5 w-5 border-t-foreground" />
             </div>
           )}
         </div>