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/AddMonthButton.tsx

@@ -22,7 +22,7 @@ export const AddMonthButton:React.FC<AddMonthButtonProps> = ({ yearMonth }) => {
                 <span className='flex self-center'>
                     <CalendarDaysIcon className="h-[1em] w-[1em] cursor-pointer text-4xl"  />
                     <PlusCircleIcon className="h-[1em] w-[1em] cursor-pointer text-xl text-green-500 ml-[-.4em] mt-[-.4em]"  />
-                    <span className="ml-1 mt-1 text-xs text-left leading-[1.2em] w-[6em]">{t("tooltip")}</span>
+                    <span className="ml-1 self-center text-xs text-left leading-[1.2em] w-[6em]">{t("tooltip")}</span>
                 </span>
             </Link>
         </div>