Fix narrow PWA layout on tablet with standalone width rules.

Allow any orientation in manifest, widen content and enable dual-column mode in installed apps on tablet viewports, with iOS display-mode fallback.

Co-authored-by: Cursor <cursoragent@cursor.com>

components/header.tsx

@@ -37,7 +37,7 @@ export default function Header() {
   return (
     <header className="relative z-10 border-b border-border/40 bg-card/70 py-3 shadow-sm backdrop-blur-md">
       <div
-        className={`${SITE_WIDTH_INNER} grid grid-cols-[1fr_auto] items-center gap-y-2 sm:flex sm:justify-between`}
+        className={`site-content-shell ${SITE_WIDTH_INNER} grid grid-cols-[1fr_auto] items-center gap-y-2 sm:flex sm:justify-between`}
       >
         <Link href="/" className="flex items-center gap-2">
           <TaijiIcon />