refactor: remove XML export, JSON-only for .rfflow files

- Remove XML builder, format query param, and XML tests
- Simplify ExportFlowModal (no format picker)
- Simplify rfflowParser (JSON-only)
- Remove format field from schemas and types

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

backend/app/schemas/tree_export.py

@@ -1,6 +1,6 @@
 """Schemas for .rfflow file export and import."""
 from datetime import datetime
-from typing import Optional, Any, Literal
+from typing import Optional, Any
 from pydantic import BaseModel, Field
 
 from app.schemas.tree import TreeType
@@ -30,7 +30,6 @@ class FlowExportEnvelope(BaseModel):
     rfflow_version: str = "1.0"
     exported_at: datetime
     source_app: str = "ResolutionFlow"
-    format: Literal["json", "xml"] = "json"
     flow: FlowExportData
 
 
@@ -39,7 +38,6 @@ class FlowImportRequest(BaseModel):
     rfflow_version: str = Field(..., description="Must be '1.0'")
     exported_at: datetime
     source_app: str = "ResolutionFlow"
-    format: Literal["json", "xml"] = "json"
     flow: FlowExportData