fix(#103): move language switcher from header into mobile nav drawer

On mobile the header is now cleaner — language buttons move to the
bottom of the hamburger panel. Desktop header is unchanged (sm:flex).

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

frontend/src/routes/+layout.svelte

@@ -46,8 +46,8 @@ const userInitials = $derived.by(() => {
 
 					<!-- Right Side -->
 					<div class="flex items-center gap-3">
-						<!-- Language selector -->
-						<div class="flex items-center gap-1 border-r border-line pr-3">
+						<!-- Language selector (desktop only — mobile lives in nav drawer) -->
+						<div class="hidden items-center gap-1 border-r border-line pr-3 sm:flex">
 							{#each locales as locale (locale)}
 								<button
 									type="button"