account icon: brand-green container with accent-yellow user glyph

Switched both the desktop and mobile account IconButton from `ghost`
to `float` (bg-brand-float + text-accent) with shadow disabled — same
md size (w-11 h-11) as the cart button beside it, so the pair reads
as a balanced complementary set: yellow cart, green account.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/design-system/components/Navbar.vue

@@ -259,7 +259,8 @@ onBeforeUnmount(() => {
         <LanguageSwitcher :tone="variant" />
         <IconButton
           icon="user"
-          variant="ghost"
+          variant="float"
+          shadow="none"
           size="md"
           :aria-label="t('auth.login.cta')"
           @click="goToLogin"
@@ -281,7 +282,8 @@ onBeforeUnmount(() => {
       <div class="md:hidden flex items-center">
         <IconButton
           icon="user"
-          variant="ghost"
+          variant="float"
+          shadow="none"
           size="md"
           :aria-label="t('auth.login.cta')"
           @click="goToLogin"