fix: address CSS quality-debt from PR #15 review feedback (#55)

- Convert tab indentation to 4 spaces in admin/css/admin-styles.css
  and admin/css/update-source-selector.css per project coding standards
- Add stylelint and stylelint-config-standard to devDependencies
- Add lint:css npm script to enable CSS quality checking
- Update quality script to include CSS linting

The playground/multisite.html CSS rule-empty-line-before fix was already
applied in commit 3ca2fe5. This PR formally closes the quality-debt
tracking issue by verifying the fix and adding CSS linting tooling to
prevent similar regressions.

Closes #38

package.json

@@ -29,6 +29,7 @@
     "test:phpunit:multisite": "WP_MULTISITE=1 composer test",
     "build": "./build.sh",
     "lint:js": "eslint cypress/",
+    "lint:css": "stylelint \"**/*.css\" --allow-empty-input",
     "lint:php": "composer run-script phpcs",
     "lint:php:simple": "composer run-script phpcs:simple",
     "lint:phpstan": "composer run-script phpstan",
@@ -38,7 +39,7 @@
     "test:php": "composer run-script test",
     "lint": "composer run-script lint",
     "fix": "composer run-script fix",
-    "quality": "npm run lint && npm run test:php"
+    "quality": "npm run lint && npm run lint:css && npm run test:php"
   },
   "repository": {
     "type": "git",
@@ -64,6 +65,8 @@
     "@wp-playground/cli": "^3.0.22",
     "cypress": "^13.17.0",
     "eslint": "^8.57.0",
-    "eslint-plugin-cypress": "^2.15.1"
+    "eslint-plugin-cypress": "^2.15.1",
+    "stylelint": "^16.0.0",
+    "stylelint-config-standard": "^36.0.0"
   }
 }