chore: bump to v0.9.19-rc.1 and add local OpenAPI build support

- Add release candidate (rc) support to release.sh with latestRC tagging
- Bump VERSION, backend pyproject.toml, and frontend package.json to 0.9.19-rc.1
- Add local frontend/openapi.json so build no longer needs running backend
- Update generate:types and validate-types.sh to use local openapi.json
- Fix frontend tests: remove unused imports/variables and update mock data

frontend/src/components/__tests__/ErrorBoundary.test.tsx

@@ -1,7 +1,6 @@
 import { describe, it, expect, vi } from "vitest";
 import { render, screen } from "@testing-library/react";
 import { ErrorBoundary } from "../ErrorBoundary";
-import * as telemetry from "../../utils/telemetry";
 
 // Mock telemetry to verify it's called
 vi.mock("../../utils/telemetry");