fix: render snake vs flow edge animations correctly

edges/index.tsx was never committed — both animation modes were
rendering as snake (truthy string check). Now uses animMode to
distinguish 'snake' (moving blob) from 'flow' (continuous flowing dashes).

frontend/src/utils/canvasSerializer.ts

@@ -41,7 +41,7 @@ export interface ApiEdge {
   speed?: string | null
   custom_color?: string | null
   path_style?: string | null
-  animated?: boolean
+  animated?: boolean | 'snake' | 'flow' | 'none'
   source_handle?: string | null
   target_handle?: string | null
 }