feat: add flow export/import frontend + backend tests

Frontend:
- ExportFlowModal with JSON/XML format selection + download
- ImportFlowModal with drag-drop file picker + preview step
- rfflowParser for client-side JSON/XML .rfflow parsing
- Export buttons on editor toolbar and library action menus
- Import button on library page next to Create New
- Provenance display for imported flows in editor
- flowTransfer API client + types

Backend:
- Fix regex->pattern deprecation in export endpoint
- 12 integration tests covering export, import, round-trip,
  access control, tag/category creation, version validation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/endpoints/tree_transfer.py

@@ -89,7 +89,7 @@ async def export_tree(
     tree_id: UUID,
     db: Annotated[AsyncSession, Depends(get_db)],
     current_user: Annotated[User, Depends(get_current_active_user)],
-    format: str = Query("json", regex="^(json|xml)$"),
+    format: str = Query("json", pattern="^(json|xml)$"),
 ):
     """Export a tree as a downloadable .rfflow file (JSON or XML)."""
     # Load tree with relationships + author name