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>

backend/app/schemas/tree.py

@@ -176,10 +176,12 @@ class TreeListResponse(BaseModel):
     category_info: Optional[CategoryInfo] = None
     tags: list[str] = []  # List of tag names
     author_id: Optional[UUID] = None
+    author_name: Optional[str] = None  # Display name or email of author
     account_id: Optional[UUID] = None
     is_active: bool
     is_public: bool
     is_default: bool
+    visibility: str = 'team'
     status: str  # draft or published
     version: int
     usage_count: int