Add withLocale() to global browser configuration

Problem

withLocale() exists on PendingAwaitablePage (per-visit level):

$page = visit('/')->withLocale('fr-FR');

But there is no equivalent on the global pest()->browser() configuration (Configuration class). The default locale is hardcoded to en-US in PendingAwaitablePage::buildAwaitablePage():

$context = $browser->newContext([
    'locale' => 'en-US',
    // ...
    ...$options,
]);

This means every visit() call defaults to en-US for navigator.language, and there's no way to change this globally without passing ['locale' => 'fr'] to every single visit() call.

Use case

Applications that detect the browser language via navigator.language to set the UI language need the Playwright browser context to use the correct locale. With 60+ test files and 179 visit() calls, passing the locale to each one is not practical.

Desired behavior

// tests/Pest.php
pest()->browser()->withLocale('fr');

Similar to how withHost(), userAgent(), timeout(), inDarkMode() etc. already work on Configuration.

Suggested implementation

1. Add Playwright::setDefaultLocale(string $locale) and Playwright::defaultLocale() (similar to existing setHost/host pattern)
2. Add Configuration::withLocale(string $locale) that calls Playwright::setDefaultLocale()
3. Use Playwright::defaultLocale() ?? 'en-US' in PendingAwaitablePage::buildAwaitablePage()

Environment

• pest-plugin-browser: 4.0
• Pest: 4.x
• PHP: 8.5

Start with the existing setHost/host pattern in Playwright and Configuration, then inspect PendingAwaitablePage::buildAwaitablePage() where the browser context locale defaults to en-US. Add the global locale behavior described in the issue and verify that visit() uses it for navigator.language while retaining en-US when no locale is configured.