feat: show login/signup buttons on mobile header

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/frontend/src/lib/components/header/header.svelte

@@ -163,7 +163,7 @@
         </div>
       {:else}
         <div class="w-full flex items-center justify-end gap-2">
-          <div class="hidden lg:flex gap-4">
+          <div class="flex gap-4">
             <Button variant="outline" class="font-medium" href="/login">{$_("header.login")}</Button>
             <Button
               href="/signup"
@@ -171,17 +171,6 @@
               >{$_("header.signup")}</Button
             >
           </div>
-          <div class="flex lg:hidden items-center gap-1 rounded-full bg-muted/30 p-1">
-            <Button
-              variant="link"
-              size="icon"
-              class="h-9 w-9 rounded-full p-0 text-foreground/80 hover:text-foreground"
-              href="/login"
-              title={$_("header.login")}
-            >
-              <span class="icon-[ri--login-circle-line] h-4 w-4"></span>
-            </Button>
-          </div>
           <div class="lg:hidden ml-2">
             <BurgerMenuButton
               label={$_("header.navigation")}