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/tree-editor/code-mode/CodeModeEditor.tsx

@@ -9,6 +9,7 @@ import { createCompletionProvider } from './resolutionFlowCompletions'
 import { CodeModeToolbar } from './CodeModeToolbar'
 import { SyntaxHelpPanel } from './SyntaxHelpPanel'
 import { setMonacoEditor } from './monacoEditorRef'
+import { Spinner } from '@/components/common/Spinner'
 
 export function CodeModeEditor() {
   const editorRef = useRef<MonacoEditor.IStandaloneCodeEditor | null>(null)
@@ -167,7 +168,7 @@ export function CodeModeEditor() {
           onMount={handleEditorDidMount}
           loading={
             <div className="flex h-full items-center justify-center bg-card">
-              <div className="h-6 w-6 animate-spin rounded-full border-2 border-border border-t-foreground" />
+              <Spinner size="sm" className="h-6 w-6 border-t-foreground" />
             </div>
           }
           options={{