feat: add Basic animation type for edges

Adds a new "basic" animation mode that uses React Flow's native animated
dashes — the entire stroke moves as dashes, with no solid line underneath.
Distinct from "snake" (moving dot) and "flow" (overlay on solid line).

- Frontend: new Basic button in EdgeModal, animMode mapping, passes
  animated={true} to BaseEdge when mode is "basic"
- Backend: normalize_animated accepts "basic" as a valid value
- Tests: EdgeModal + canvas round-trip tests for basic mode

frontend/src/types/index.ts

@@ -107,7 +107,7 @@ export interface EdgeData extends Record<string, unknown> {
   speed?: string
   custom_color?: string
   path_style?: EdgePathStyle
-  animated?: boolean | 'snake' | 'flow' | 'none'
+  animated?: boolean | 'snake' | 'flow' | 'basic' | 'none'
   waypoints?: Waypoint[]
 }