feat: add edge flow animation (dot traveling source→target)

- Add animated toggle per edge in EdgeModal (cyan switch, "Flow Animation")
- SVG-native <animate> element for reliable cross-browser dot animation
- Persist animated field: backend model, schemas (EdgeBase/EdgeUpdate/EdgeSave), DB migration
- Include animated in App.tsx edgesToSave serialization so it survives save/reload
- Add animated: bool to EdgeData TypeScript type

backend/app/schemas/canvas.py

@@ -37,6 +37,7 @@ class EdgeSave(BaseModel):
     speed: str | None = None
     custom_color: str | None = None
     path_style: str | None = None
+    animated: bool = False
     source_handle: str | None = None
     target_handle: str | None = None