[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

package.json

@@ -36,7 +36,7 @@
 		"config:check": "node js/check_config.js",
 		"postinstall": "git clean -df fonts vendor",
 		"install-mm": "npm install --no-audit --no-fund --no-update-notifier --only=prod --omit=dev",
-		"install-mm:dev": "npm install --no-audit --no-fund --no-update-notifier",
+		"install-mm:dev": "npm install --no-audit --no-fund --no-update-notifier && npx playwright install chromium",
 		"lint:css": "stylelint 'css/main.css' 'css/roboto.css' 'css/font-awesome.css' 'modules/default/**/*.css' --fix",
 		"lint:js": "eslint --fix",
 		"lint:markdown": "markdownlint-cli2 . --fix",
@@ -106,6 +106,7 @@
 		"eslint-plugin-import-x": "^4.16.1",
 		"eslint-plugin-jsdoc": "^61.1.11",
 		"eslint-plugin-package-json": "^0.59.1",
+		"eslint-plugin-playwright": "^2.3.0",
 		"eslint-plugin-vitest": "^0.5.4",
 		"express-basic-auth": "^1.2.1",
 		"husky": "^9.1.7",