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

backend/pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "bangui-backend"
-version = "0.9.19"
+version = "0.9.19-rc.1"
 description = "BanGUI backend — fail2ban web management interface"
 requires-python = ">=3.12"
 dependencies = [