feat(LanguageSwitcher): add inverted prop for dark-header context

When inverted=true, buttons render white text instead of ink tokens,
suitable for placement on brand-navy background.

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

frontend/src/lib/components/LanguageSwitcher.svelte

@@ -1,6 +1,8 @@
 <script lang="ts">
 import { setLocale, getLocale } from '$lib/paraglide/runtime';
 
+let { inverted = false }: { inverted?: boolean } = $props();
+
 const locales = ['DE', 'EN', 'ES'] as const;
 const localeMap = { DE: 'de', EN: 'en', ES: 'es' } as const;
 const activeLocale = $derived(getLocale().toUpperCase());
@@ -11,7 +13,13 @@ const activeLocale = $derived(getLocale().toUpperCase());
 		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'}"
+               {activeLocale === locale
+			? inverted
+				? 'font-bold text-white'
+				: 'font-bold text-ink'
+			: inverted
+				? 'font-normal text-white/55 hover:text-white/85'
+				: 'font-normal text-ink-3 hover:text-ink'}"
 	>
 		{locale}
 	</button>