chore: update test fixtures and frontend for UUID identity (refs #217)

Backend test fixtures:
- Replace auth0|xxx format with UUID in all test userId values
- Update admin tests for new id/userProfileId schema
- Add missing deletionRequestedAt/deletionScheduledFor to auth test mocks
- Fix admin integration test supertest usage (app.server)

Frontend:
- AdminUser type: auth0Sub -> id + userProfileId
- admin.api.ts: all user management methods use userId (UUID) params
- useUsers/useAdmins hooks: auth0Sub -> userId/id in mutations
- AdminUsersPage + AdminUsersMobileScreen: user.auth0Sub -> user.id
- Remove encodeURIComponent (UUIDs don't need encoding)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/features/maintenance/tests/fixtures/maintenance.fixtures.json

@@ -99,7 +99,7 @@
   },
   "maintenanceScheduleResponse": {
     "id": "f47ac10b-58cc-4372-a567-0e02b2c3d479",
-    "userId": "auth0|user123",
+    "userId": "550e8400-e29b-41d4-a716-446655440000",
     "vehicleId": "550e8400-e29b-41d4-a716-446655440000",
     "type": "oil_change",
     "category": "routine_maintenance",