Replace Java-based OpenAPI generator with Node-native alternative

[pkarlsson-codes]

Problem

generate-api-from-spec (and related scripts) use @openapitools/openapi-generator-cli, which is a wrapper around the Java OpenAPI Generator JAR. This means Java must be installed to generate the TypeScript API client, even though the rest of the toolchain is pure Node/npm.

This blocks contributors on Linux (and CI environments) that don't have Java installed.

Proposed Solution

Replace @openapitools/openapi-generator-cli with a Node-native alternative. Leading candidates:

• openapi-typescript + openapi-fetch — generates TypeScript types from the spec; pairs with a lightweight fetch wrapper. Actively maintained, no runtime class bloat.
• hey-api/openapi-ts — generates typed clients, also Node-native.

Note: openapi-typescript-codegen is already in dependencies but generates a different class-based structure and is largely unmaintained — it should probably be removed rather than used as the replacement.

Acceptance Criteria

• npm run frontend:generate (and related generate scripts) work without Java installed
• Generated client is consumed by the frontend without breaking existing usage
• openapi-typescript-codegen and @openapitools/openapi-generator-cli removed from package.json
• generate-api-clients.sh updated accordingly