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

backend/app/schemas/utils.py

@@ -4,6 +4,6 @@ def normalize_animated(v: object) -> str:
         return 'snake'
     if v is False or v == 0 or v == '0' or v is None or v == 'none':
         return 'none'
-    if v in ('snake', 'flow'):
+    if v in ('snake', 'flow', 'basic'):
         return str(v)
     return 'none'