fix: stabilize case step editor state

[makoto-soracom]

Summary

• extract only the case step editor fix from merge-upstream-v1.0.0-beta.28
• prevent inserted, deleted, and edited steps from overwriting adjacent step state
• refresh fetched case data after save so the editor returns to a stable server-backed state

Key changes

• move step insertion, deletion, replacement, and fetched-step normalization into frontend/utils/stepEditorState.ts
• switch the rendered step row key in CaseStepsEditor from an index-based key to a stable uid
• keep idCounter aligned with fetched steps and add focused tests for the step editor state helper

Tests

• npm test -- --run frontend/utils/stepEditorState.test.ts (fails in this environment: vitest: not found)