Add integration tests for Chrome extension #17
Description
What
Add automated tests for the Chrome extension code. Currently all 30 tests cover the Python server - the extension JavaScript has no test coverage.
Scope
- Set up a JS testing framework (Jest or Vitest)
- Test `content.js`: DOM observation, text extraction, treatment application
- Test `background.js`: API proxying, health check logic, settings management
- Test `popup.js`: settings toggle behavior, slider updates
- Mock `chrome.storage` and `chrome.runtime` APIs
Why
The extension is the user-facing surface of the project. Changes to DOM selectors, treatment logic, or settings handling should be caught by tests before reaching users.
References
- `extension/content.js`, `extension/background.js`, `extension/popup/popup.js`
- Chrome Extension Testing docs