[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-lock.json

@@ -44,6 +44,7 @@
 				"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",
@@ -5496,6 +5497,35 @@
 				"jsonc-eslint-parser": "^2.0.0"
 			}
 		},
+		"node_modules/eslint-plugin-playwright": {
+			"version": "2.3.0",
+			"resolved": "https://registry.npmjs.org/eslint-plugin-playwright/-/eslint-plugin-playwright-2.3.0.tgz",
+			"integrity": "sha512-7UeUuIb5SZrNkrUGb2F+iwHM97kn33/huajcVtAaQFCSMUYGNFvjzRPil5C0OIppslPfuOV68M/zsisXx+/ZvQ==",
+			"dev": true,
+			"license": "MIT",
+			"dependencies": {
+				"globals": "^16.4.0"
+			},
+			"engines": {
+				"node": ">=16.9.0"
+			},
+			"peerDependencies": {
+				"eslint": ">=8.40.0"
+			}
+		},
+		"node_modules/eslint-plugin-playwright/node_modules/globals": {
+			"version": "16.5.0",
+			"resolved": "https://registry.npmjs.org/globals/-/globals-16.5.0.tgz",
+			"integrity": "sha512-c/c15i26VrJ4IRt5Z89DnIzCGDn9EcebibhAOjw5ibqEHsE1wLUgkPn9RDmNcUKyU87GeaL633nyJ+pplFR2ZQ==",
+			"dev": true,
+			"license": "MIT",
+			"engines": {
+				"node": ">=18"
+			},
+			"funding": {
+				"url": "https://github.com/sponsors/sindresorhus"
+			}
+		},
 		"node_modules/eslint-plugin-vitest": {
 			"version": "0.5.4",
 			"resolved": "https://registry.npmjs.org/eslint-plugin-vitest/-/eslint-plugin-vitest-0.5.4.tgz",