feat: implement layout 3-tier container architecture, desktop screen token, Gate 8 no-escape rule, and clean dead code

refer_landing_page/components/Header.tsx

@@ -28,7 +28,7 @@ export default function Header() {
   return (
     <header className="sticky top-0 z-50 border-b border-ink bg-ivory/85 backdrop-blur">
       {/* Journal issue strip — magazine masthead feel. */}
-      <div className="hidden border-b border-line lg:block">
+      <div className="hidden border-b border-line desktop:block">
         <div className="container-content flex items-center justify-between py-1.5">
           <span className="kicker">Issue 01 — 2026</span>
           {/* CUSTOMIZATION HOOK: masthead tagline */}
@@ -55,7 +55,7 @@ export default function Header() {
         </Link>
 
         {/* Desktop nav — small caps, wide tracking (visible at >= 1240px) */}
-        <div className="hidden items-center gap-7 min-[1240px]:flex">
+        <div className="hidden items-center gap-7 desktop:flex">
           <nav className="flex items-center gap-7" aria-label="주 메뉴">
             {navItems.map((item) => (
               <Link
@@ -85,7 +85,7 @@ export default function Header() {
         </div>
 
         {/* Mobile controls (visible under 1240px) */}
-        <div className="flex items-center gap-2 min-[1240px]:hidden">
+        <div className="flex items-center gap-2 desktop: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 min-[1240px]:hidden"
+          className="border-t border-ink bg-ivory desktop:hidden"
           aria-label="모바일 메뉴"
         >
           <ul className="container-content flex flex-col divide-y divide-line py-2">