[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/serveronly_spec.js

@@ -38,11 +38,13 @@ describe("App environment", () => {
 describe("Check config", () => {
 	it("config check should return without errors", async () => {
 		process.env.MM_CONFIG_FILE = "tests/configs/default.js";
-		await expect(runConfigCheck()).resolves.toBe(0);
+		const exitCode = await runConfigCheck();
+		expect(exitCode).toBe(0);
 	});
 
 	it("config check should fail with non existent config file", async () => {
 		process.env.MM_CONFIG_FILE = "tests/configs/not_exists.js";
-		await expect(runConfigCheck()).resolves.toBe(1);
+		const exitCode = await runConfigCheck();
+		expect(exitCode).toBe(1);
 	});
 });