Restore stealth plugin and fix fetcher language configuration

[clementbiron]

This PR reactivates puppeteer-extra-plugin-stealth in the full DOM fetcher. Since v10.3.1 (commit 5e4945d2), puppeteer.use(stealthPlugin(...)) was called inside fetch() instead of before puppeteer.launch(), so puppeteer-extra never bound its onPageCreated hooks and the plugin was silently inert. navigator.webdriver and HeadlessChrome were exposed to every tracked service for around four months.

It also applies the @opentermsarchive/engine.fetcher.language configuration to navigator.language and navigator.languages through the stealth sub-evasions, not just the Accept-Language HTTP header that was already affected. The stealth wrapper's locale option was being silently ignored. The redundant setExtraHTTPHeaders and CDP Network.setUserAgentOverride calls in configurePage are removed: they duplicated stealth's path with subtly different results.

Breaking: quality factors (;q=...) are no longer accepted in @opentermsarchive/engine.fetcher.language. Previously accepted values such as en-IE,en-GB;q=0.9,en;q=0.8 now throw at launchHeadlessBrowser. Combined with the now-removed setExtraHTTPHeaders call, those values used to produce malformed headers like q=0.9;q=0.9, and the surface area was not worth preserving. Provide a plain comma-separated priority list instead (e.g. en-IE,en-GB,en); the browser derives the Accept-Language quality factors from tag order.

As a side effect of applying the configuration to navigator.languages, language: "en" now exposes navigator.languages as ["en"] instead of the previous default ["en-US", "en"]. Setting language: "en-US,en" restores the previous default.