Center button labels vertically using self-center

Replace fixed top margin with self-center for vertical alignment
in PrintButton, AddLocationButton, and AddMonthButton text labels.

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

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

app/ui/PrintButton.tsx

@@ -24,7 +24,7 @@ export const PrintButton: React.FC<PrintButtonProps> = ({ yearMonth }) => {
       >
         <span className='flex self-center'>
           <PrinterIcon className="h-[1em] w-[1em] cursor-pointer text-4xl" />
-          <span className="ml-1 mt-[.4em] text-xs text-left leading-[1.2em] w-[6em]">{t("print-codes-label")}</span>
+          <span className="ml-1 self-center text-xs text-left leading-[1.2em] w-[6em]">{t("print-codes-label")}</span>
         </span>
       </button>
     </div>