Issue #3627123: Separate dashboard custom properties from declarations

Follow-up to #3627123 and !7 (merged). Pipeline 982998 failed stylelint at css/dashboard.css:20 because declarations were not separated from custom properties by a blank line. Add that blank line; CSS behavior is unchanged.

Pinned Stylelint 16.26.1 with Drupal core configuration reproduces the baseline error and passes both admin.css and dashboard.css with this change. No rules are suppressed. Independent review found no issues.

Hosted merged-results pipeline 983242 confirms stylelint, PHPCS, PHPStan, ESLint, cspell, both Composer jobs and default PHPUnit pass. Previous-minor PHPUnit still fails testEnglishWorkingCopyPreservedWhenAddingTranslation with HTTP 409 (translation already exists) instead of 200. The same failure reproduced in the earlier same-source retry, job12550363. This formatting correction does not resolve that separate runtime defect; its contained reproduction remains in progress.