fix(form): inverted label color in grouped fields

Added the `.grouped.fields > label` selector for targeted styling, ensuring labels within grouped fields inherit the inverted color in inverted forms. Used `@invertedLabelColor` for color consistency. Tested in various configurations and browsers.

Author: Festiis

src/definitions/collections/form.less

@@ -866,6 +866,7 @@
     .ui.form .inverted.segment label,
     .ui.form .inverted.segment .ui.checkbox label,
     .ui.inverted.form .ui.checkbox label,
+    .ui.inverted.form .grouped.fields > label,
     .ui.inverted.form .inline.fields > label,
     .ui.inverted.form .inline.fields .field > label,
     .ui.inverted.form .inline.fields .field > p,