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/api/client.ts

@@ -11,8 +11,8 @@ export const apiClient = axios.create({
   },
 })
 
-// Global error handler - shows toast for common API errors
-// Pages can still catch errors explicitly if they need custom handling
+// Global error handler for shared cases only.
+// By convention, 4xx errors are handled at the page/component level.
 function handleGlobalError(error: AxiosError) {
   // Network error (no response from server)
   if (!error.response) {
@@ -43,9 +43,8 @@ function handleGlobalError(error: AxiosError) {
     return
   }
 
-  // Client errors (4xx) — don't toast globally.
-  // Pages handle their own 4xx errors (permission checks, validation, not-found)
-  // and many are caught silently. Global toasts here cause noisy duplicates.
+  // Client errors (4xx) remain page-owned to avoid duplicate/noisy toasts.
+  // Global handling only covers 401/429/5xx.
   if (status >= 400 && status < 500) {
     return
   }