Add choices API endpoint and OpenAPI client setup

Introduces a new /api/choices/ endpoint for fetching model choices with multilingual labels. Updates Django models to use 'cz#' prefix for Czech labels. Adds OpenAPI client generation via orval, refactors frontend API structure, and provides documentation and helper scripts for dynamic choices and OpenAPI usage.

.github/copilot-instructions.md

@@ -107,6 +107,13 @@ Notes
 - **Task queue**: Celery + Redis for async/background jobs.
 - **API**: REST endpoints, JWT auth, API key support.
 
+### OpenAPI Client Generation
+- Schema: `config = { schemaUrl: "/api/schema/", baseUrl: "/api/" }`
+- Commands: `npm run api:update` (fetch schema + generate client)
+- Output: `frontend/src/api/generated/` (TypeScript Axios client)
+- Axios instance: `frontend/src/api/api.ts` with `withCredentials` and JWT auto-refresh via existing `Client.ts`.
+- Choices helper: `frontend/src/api/get_choices.ts` → `getChoices(requests, lang)` returns `{ "Model.field": [{ value, label }] }`.
+
 ## References
 - [frontend/REACT.md](../frontend/REACT.md): Frontend structure, workflows, and conventions.
 - [frontend/src/layouts/LAYOUTS.md](../frontend/src/layouts/LAYOUTS.md): Layout/component patterns.