Give the xs button M3's 16dp padding, and stop groups wrapping

ButtonXSmallTokens puts 16dp either side of an xs label, where the button, the
<x-group> segment and --group-pad all wrote 12px; the variants table calls 16dp
the Expressive value twice over. A standard button group also dropped onto a
second line on a narrow window, which M3 forbids — and the press expansion only
ever reaches a neighbour on the same line. Plan step 18, actions.md ACT-09,
ACT-10.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

tests/Feature/Components/ButtonTest.php

@@ -49,7 +49,7 @@ it('sizes a label button on Expressive\'s scale', function (string $size, string
 
     expect($html)->toContain($classes)->toContain($icon);
 })->with([
-    'xs' => ['xs', 'h-8 gap-2 px-3 type-label-lg', 'size-5'],
+    'xs' => ['xs', 'h-8 gap-2 px-4 type-label-lg', 'size-5'],
     'sm' => ['sm', 'h-10 gap-2 px-4 type-label-lg', 'size-5'],
     'md' => ['md', 'h-14 gap-2 px-6 type-title-md', 'size-6'],
     'lg' => ['lg', 'h-24 gap-3 px-12 type-headline-sm', 'size-8'],