Adds server-side validation for the `unavailableNodePolicy` field on the
PUT /api/settings route. Accepts only `block` or `fallback-local`;
rejects unknown strings and non-string values with 400.
- packages/dashboard/src/routes/register-settings-memory-routes.ts:
call `validateUnavailableNodePolicy` and throw `badRequest` on invalid input
- packages/dashboard/src/__tests__/routes.test.ts: cover accept,
invalid-value, and non-string cases
Fusion-Task-Id: FN-2952
7afbd553c2