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/TreeEditorPage.tsx

@@ -651,7 +651,7 @@ export function TreeEditorPage() {
           {/* Publish */}
           <button
             onClick={handlePublish}
-            disabled={isSaving || !isDirty || hasBlockingErrors}
+            disabled={isSaving || hasBlockingErrors}
             title={hasBlockingErrors ? 'Fix validation errors before publishing (Ctrl+S when no errors)' : 'Publish tree (Ctrl+S when no errors)'}
             className={cn(
               'flex items-center gap-2 rounded-md bg-gradient-brand px-4 py-2 text-sm font-medium text-white shadow-lg shadow-primary/20',