feat: implement 4 future backlog tasks (side margins, max-width, 1240px header breakpoint, remove intl-conf category)

refer_landing_page/components/Header.tsx

@@ -54,8 +54,8 @@ export default function Header() {
           </span>
         </Link>
 
-        {/* Desktop nav — small caps, wide tracking */}
-        <div className="hidden items-center gap-7 lg:flex">
+        {/* Desktop nav — small caps, wide tracking (visible at >= 1240px) */}
+        <div className="hidden items-center gap-7 min-[1240px]:flex">
           <nav className="flex items-center gap-7" aria-label="주 메뉴">
             {navItems.map((item) => (
               <Link
@@ -84,8 +84,8 @@ export default function Header() {
           <ThemeToggle />
         </div>
 
-        {/* Mobile controls */}
-        <div className="flex items-center gap-2 lg:hidden">
+        {/* Mobile controls (visible under 1240px) */}
+        <div className="flex items-center gap-2 min-[1240px]:hidden">
           <ThemeToggle />
           <button
             type="button"
@@ -110,7 +110,7 @@ export default function Header() {
       {open && (
         <nav
           id="mobile-menu"
-          className="border-t border-ink bg-ivory lg:hidden"
+          className="border-t border-ink bg-ivory min-[1240px]:hidden"
           aria-label="모바일 메뉴"
         >
           <ul className="container-content flex flex-col divide-y divide-line py-2">