feat: replace mobile header logo image with i18n text

Use ArkLogoMark + t('brand') on the mobile header so the wordmark can be translated (zh-CN: 'ARK 资料库', en: 'ARK Library'). Desktop nav already used this pattern.

src/layouts/PublicLayout.tsx

@@ -278,18 +278,11 @@ export function PublicLayout() {
         <div className="flex h-[64px] items-center justify-between bg-[#08070c] px-[20px] py-[12px] md:hidden">
           <Link
             to="/"
-            className="flex h-[28px] w-[160px] shrink-0 rounded-sm outline-none focus-visible:ring-2 focus-visible:ring-ark-gold/80 focus-visible:ring-offset-2 focus-visible:ring-offset-[#08070c]"
+            className="flex h-[28px] shrink-0 items-center gap-[8px] rounded-sm text-[18px] font-bold tracking-wide text-ark-gold outline-none focus-visible:ring-2 focus-visible:ring-ark-gold/80 focus-visible:ring-offset-2 focus-visible:ring-offset-[#08070c]"
             aria-label={t("brand")}
           >
-            <img
-              src="/assets/ark-library/header-logo.svg"
-              alt="ARK LIBRARY"
-              className="h-full w-full object-contain"
-              width={160}
-              height={28}
-              decoding="async"
-              draggable={false}
-            />
+            <ArkLogoMark className="h-[28px] w-[28px] shrink-0" />
+            <span className="truncate text-ark-gold">{t("brand")}</span>
           </Link>
 
           <div className="flex h-[40px] w-[136px] shrink-0 items-center gap-[8px]">