🚚(frontend) rename folder app to apps

The folder app will be used for more than one app, so it was
renamed to apps.

src/frontend/apps/desk/stylelint.config.js

@@ -0,0 +1,8 @@
+module.exports = {
+  extends: ['stylelint-config-standard', 'stylelint-prettier/recommended'],
+  rules: {
+    'custom-property-pattern': null,
+    'selector-class-pattern': null,
+    'no-descending-specificity': null,
+  },
+};