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

Docker/Dockerfile.frontend

@@ -18,7 +18,7 @@ WORKDIR /build
 COPY frontend/package.json frontend/package-lock.json* /build/
 RUN npm ci --ignore-scripts
 
-# Copy source and build
+# Copy source + local OpenAPI spec (avoids needing a running backend during build)
 COPY frontend/ /build/
 RUN npm run build