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/types/tree.ts

@@ -164,6 +164,7 @@ export interface TreeListItem {
   category_info: CategoryInfo | null
   tags: string[]
   author_id: string | null
+  author_name: string | null
   account_id: string | null
   is_active: boolean
   is_public: boolean
@@ -173,6 +174,7 @@ export interface TreeListItem {
   usage_count: number
   created_at: string
   updated_at: string
+  visibility: 'private' | 'team' | 'link' | 'public'
 }
 
 export interface TreeCreate {
@@ -215,6 +217,7 @@ export interface TreeFilters {
   is_active?: boolean
   author_id?: string
   is_public?: boolean
+  visibility?: 'private' | 'team' | 'link' | 'public'
   sort_by?: 'usage_count' | 'updated_at' | 'created_at' | 'name' | 'name_desc' | 'version'
   skip?: number
   limit?: number

frontend/src/types/user.ts

@@ -8,8 +8,8 @@ export interface User {
   is_super_admin: boolean
   is_active: boolean
   must_change_password: boolean
-  account_id: string
-  account_role: 'owner' | 'engineer' | 'viewer'
+  account_id: string | null
+  account_role: 'owner' | 'engineer' | 'viewer' | null
   created_at: string
   last_login: string | null
 }