Fix issue #4103: Improve description of how to do frontend setup and testing in `.openhands_instructions` (#4104)

Co-authored-by: Graham Neubig <neubig@gmail.com>

.openhands_instructions

@@ -1,7 +1,27 @@
 OpenHands is an automated AI software engineer. It is a repo with a Python backend
-(in the `openhands` directory) and typescript frontend (in the `frontend` directory).
+(in the `openhands` directory) and TypeScript frontend (in the `frontend` directory).
 
-- Setup: To set up the repo, including frontend/backend you can `make build`
-- Backend Testing: All tests are in `tests/unit/test_*.py`. To test new code, you
-  can do `poetry run pytest tests/unit/test_xxx.py` where `xxx` is the appropriate
-  file for the current functionality. Write all tests with pytest.
+General Setup:
+- To set up the entire repo, including frontend and backend, run `make build`
+
+Backend:
+- Located in the `openhands` directory
+- Testing:
+  - All tests are in `tests/unit/test_*.py`
+  - To test new code, run `poetry run pytest tests/unit/test_xxx.py` where `xxx` is the appropriate file for the current functionality
+  - Write all tests with pytest
+
+Frontend:
+- Located in the `frontend` directory
+- Prerequisites: A recent version of NodeJS / NPM
+- Setup: Run `npm install` in the frontend directory
+- Testing:
+  - Run tests: `npm run test`
+  - To run specific tests: `npm run test -- -t "TestName"`
+- Building:
+  - Build for production: `npm run build`
+- Environment Variables:
+  - Set in `frontend/.env` or as environment variables
+  - Available variables: VITE_BACKEND_HOST, VITE_USE_TLS, VITE_INSECURE_SKIP_VERIFY, VITE_FRONTEND_PORT
+- Internationalization:
+  - Generate i18n declaration file: `npm run make-i18n`