refactor: extract LanguageSwitcher into a reusable component

Removes duplicated locale logic from +layout.svelte and AppNav.svelte.
Context-specific sizing (text-xs/min-h-[44px]) stays in the wrapper
via [&_button]: selectors so the component itself is layout-agnostic.

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

frontend/src/lib/components/LanguageSwitcher.svelte

@@ -0,0 +1,18 @@
+<script lang="ts">
+import { setLocale, getLocale } from '$lib/paraglide/runtime';
+
+const locales = ['DE', 'EN', 'ES'] as const;
+const localeMap = { DE: 'de', EN: 'en', ES: 'es' } as const;
+const activeLocale = $derived(getLocale().toUpperCase());
+</script>
+
+{#each locales as locale (locale)}
+	<button
+		type="button"
+		onclick={() => setLocale(localeMap[locale])}
+		class="font-sans tracking-widest transition-colors
+               {activeLocale === locale ? 'font-bold text-ink' : 'font-normal text-ink-3 hover:text-ink'}"
+	>
+		{locale}
+	</button>
+{/each}