feat(login): replace floating lang switcher with AuthHeader

Removes the absolutely-positioned language switcher div and replaces it
with the shared AuthHeader component (logo + lang switcher on navy bar).

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

frontend/src/routes/login/+page.svelte

@@ -1,34 +1,16 @@
 <script lang="ts">
 import { m } from '$lib/paraglide/messages.js';
-import { setLocale, getLocale } from '$lib/paraglide/runtime';
+import AuthHeader from '../AuthHeader.svelte';
 
 let { form }: { form?: { error?: string; success?: boolean } } = $props();
-
-const locales = ['DE', 'EN', 'ES'] as const;
-const localeMap = { DE: 'de', EN: 'en', ES: 'es' } as const;
-const activeLocale = $derived(getLocale().toUpperCase());
 </script>
 
 <svelte:head>
 	<title>{m.page_title_login()}</title>
 </svelte:head>
 
-<div class="relative flex min-h-screen flex-col bg-canvas">
-	<!-- Language switcher -->
-	<div class="absolute top-4 right-4 flex items-center gap-1">
-		{#each locales as locale (locale)}
-			<button
-				type="button"
-				onclick={() => setLocale(localeMap[locale])}
-				class="px-1.5 py-1 font-sans text-xs tracking-widest transition-colors
-                    {activeLocale === locale
-                        ? 'font-bold text-ink'
-                        : 'font-normal text-ink-3 hover:text-ink'}"
-			>
-				{locale}
-			</button>
-		{/each}
-	</div>
+<div class="flex min-h-screen flex-col bg-canvas">
+	<AuthHeader />
 
 	<div class="flex flex-1 items-center justify-center px-4">
 		<div class="w-full max-w-sm">