[core] refactor: replace XMLHttpRequest with fetch and migrate e2e tests to Playwright (#3950)

### 1. Replace `XMLHttpRequest` with the modern `fetch` API for loading
translation files

#### Changes
- **translator.js**: Use `fetch` with `async/await` instead of XHR
callbacks
- **loader.js**: Align URL handling and add error handling (follow-up to
fetch migration)
- **Tests**: Update infrastructure for `fetch` compatibility

#### Benefits
- Modern standard API
- Cleaner, more readable code
- Better error handling and fallback mechanisms

### 2. Migrate e2e tests to Playwright

This wasn't originally planned for this PR, but is related. While
investigating suspicious log entries which surfaced after the fetch
migration I kept running into JSDOM’s limitations. That pushed me to
migrate the E2E suite to Playwright instead.

#### Changes
- switch e2e harness to Playwright (`tests/e2e/helpers/global-setup.js`)
- rewrite specs to use Playwright locators + shared `expectTextContent`
- install Chromium via `npx playwright install --with-deps` in CI

#### Benefits
- much closer to real browser behaviour
- and no more fighting JSDOM’s quirks

tests/unit/classes/translator_spec.js

@@ -13,6 +13,7 @@ function createTranslationTestEnvironment () {
 	const dom = new JSDOM("", { url: "http://localhost:3001", runScripts: "outside-only" });
 
 	dom.window.Log = { log: vi.fn(), error: vi.fn() };
+	dom.window.fetch = fetch;
 	dom.window.eval(translatorJs);
 
 	return { window: dom.window, Translator: dom.window.Translator };