[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/e2e/helpers/weather-functions.js

@@ -1,18 +1,6 @@
 const { injectMockData, cleanupMockData } = require("../../utils/weather_mocker");
 const helpers = require("./global-setup");
 
-exports.getText = async (element, result) => {
-	const elem = await helpers.waitForElement(element);
-	expect(elem).not.toBeNull();
-	expect(
-		elem.textContent
-			.trim()
-			.replace(/(\r\n|\n|\r)/gm, "")
-			.replace(/[ ]+/g, " ")
-	).toBe(result);
-	return true;
-};
-
 exports.startApplication = async (configFileName, additionalMockData) => {
 	await helpers.startApplication(injectMockData(configFileName, additionalMockData));
 	await helpers.getDocument();