fix(ui-buttons): fix textAlign center on Buttons without icon

The content wrapper changed from display:block to display:flex in the v2
button rewrite, which made textAlign:center ineffective on the shrunk
children span. Add justifyContent based on textAlign so text is centered
regardless of whether an icon is present.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: balzss

packages/ui-buttons/src/BaseButton/v2/styles.ts

@@ -496,6 +496,7 @@ const generateStyle = (
       width: '100%',
       display: 'flex',
       alignItems: 'center',
+      justifyContent: textAlign === 'center' ? 'center' : 'flex-start',
       direction: 'inherit',
       userSelect: 'none',
       transition: 'background 0.2s, transform 0.2s',