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/ButtonGroupTest.php

@@ -17,7 +17,11 @@ it('groups buttons with the spacing of their size', function () {
 it('connects buttons 2px apart', function () {
     expect((string) $this->blade('<x-button-group connected><x-button label="Day" /></x-button-group>'))
         ->toContain('data-button-group="connected"')
-        ->toContain('gap-0.5')
+        ->toContain('gap-0.5');
+});
+
+it('never wraps a group onto a second line', function () {
+    expect((string) $this->blade('<x-button-group><x-button label="Day" /></x-button-group>'))
         ->not->toContain('flex-wrap');
 });