[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/env_spec.js

@@ -1,9 +1,13 @@
+const { expect } = require("playwright/test");
 const helpers = require("./helpers/global-setup");
 
 describe("App environment", () => {
+	let page;
+
 	beforeAll(async () => {
 		await helpers.startApplication("tests/configs/default.js");
 		await helpers.getDocument();
+		page = helpers.getPage();
 	});
 	afterAll(async () => {
 		await helpers.stopApplication();
@@ -20,8 +24,6 @@ describe("App environment", () => {
 	});
 
 	it("should show the title MagicMirror²", async () => {
-		const elem = await helpers.waitForElement("title");
-		expect(elem).not.toBeNull();
-		expect(elem.textContent).toBe("MagicMirror²");
+		await expect(page).toHaveTitle("MagicMirror²");
 	});
 });