fix(themes): cover .group elements in Flat nav row spacing rules at narrow widths (#8732)

At max-width: 840px, the existing rules only adjusted .stick wrappers
and their child buttons. .group elements were missing from the
selector list, so their child buttons kept the default 10px horizontal
margin from the .nav_menu .btn rule, breaking the visual joining of
multi-button groups (e.g. Views) at narrow widths.

Add .nav_menu .group to both existing selectors so .group is treated
the same as .stick — the wrapper gets outer margin, child buttons get
zero horizontal margin.

Same pattern already used by Alt-Dark and Pafat at narrow widths.

.rtl.css regenerated via make rtl. npm run stylelint passes.

Co-authored-by: Bjørn A. Andersen <polybjorn@users.noreply.github.com>

p/themes/Flat/flat.css

@@ -978,11 +978,13 @@ th {
 		min-height: 0;
 	}
 
-	.nav_menu .stick {
+	.nav_menu .stick,
+	.nav_menu .group {
 		margin: 0 10px;
 	}
 
-	.nav_menu .stick .btn {
+	.nav_menu .stick .btn,
+	.nav_menu .group .btn {
 		margin: 5px 0;
 	}
 

p/themes/Flat/flat.rtl.css

@@ -978,11 +978,13 @@ th {
 		min-height: 0;
 	}
 
-	.nav_menu .stick {
+	.nav_menu .stick,
+	.nav_menu .group {
 		margin: 0 10px;
 	}
 
-	.nav_menu .stick .btn {
+	.nav_menu .stick .btn,
+	.nav_menu .group .btn {
 		margin: 5px 0;
 	}