Issue #3593003: Fix CI validation failures

Summary

• Fix PHPCS/PHPStan issues in the dropdown language block by using the injected route match service and null coalescing.
• Format the admin CSS for Drupal core stylelint/prettier rules.
• Fix cspell findings in help text and tests, and add the project dictionary entry for the contributor username.
• Update restricted-path test fixtures to avoid accented alias spelling issues and only assert the block label on accessible pages.

Testing

• cspell: passes, 19 files checked, 0 issues.
• phpcs: passes with Drupal standard, 0 errors.
• phpstan: passes, no errors.
• stylelint: passes, 1 source checked, 0 problems.
• phpunit: passes, PHPUnit 11.5.55 PHP 8.3.31.

Closes #3593003