[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

CHANGELOG.md

@@ -32,6 +32,8 @@ planned for 2026-01-01
 - [tests] replace `node-libgpiod` with `serialport` in electron-rebuild workflow (#3945)
 - [calendar] hide repeatingCountTitle if the event count is zero (#3949)
 - [core] configure cspell to check default modules only and fix typos (#3955)
+- [core] refactor: replace `XMLHttpRequest` with `fetch` in `translator.js` (#3950)
+- [tests] migrate e2e tests to Playwright (#3950)
 
 ### Fixed