ui: prevent layout shift when mobile search overlay opens

- SearchPanel: focus input with { preventScroll: true } so the browser
  doesn't auto-scroll the underlying page when the overlay mounts.
- SearchPanel: add overscroll-contain on the scroll container so
  reaching the panel's edges does not chain into the body on Android.
- PublicLayout: lock background scroll while the mobile search overlay
  is open using the iOS-compatible position-fixed + restore pattern,
  so the page underneath cannot move at all.

src/components/SearchPanel.tsx

@@ -70,7 +70,11 @@ export function SearchPanel({
   const langParam = useMemo(() => langQuery(lang), [lang]);
 
   useEffect(() => {
-    inputRef.current?.focus();
+    // Avoid scroll-into-view: browsers default-scroll the focused element
+    // into the viewport, which moves the underlying page when the search
+    // overlay opens from a scrolled position. `preventScroll` keeps the page
+    // exactly where it was.
+    inputRef.current?.focus({ preventScroll: true });
   }, []);
 
   useEffect(() => {
@@ -133,7 +137,7 @@ export function SearchPanel({
   };
 
   return (
-    <div className="ark-header-menu-enter fixed inset-x-0 bottom-0 top-[64px] z-50 overflow-y-auto bg-[#0f0f13] md:hidden">
+    <div className="ark-header-menu-enter fixed inset-x-0 bottom-0 top-[64px] z-50 overflow-y-auto overscroll-contain bg-[#0f0f13] md:hidden">
       <div className="border-t border-white/10 px-5 pb-6 pt-3 max-[360px]:px-3">
         <div className="flex h-12 items-center gap-2">
           <div className="flex h-11 min-w-0 flex-1 items-center gap-2 rounded-full border border-ark-gold bg-[#191921] px-3 shadow-[0_0_0_2px_rgba(245,180,53,0.12)]">