refactor: unify recipe routes into [recipeLang] slug with full bilingual support

Consolidate /rezepte and /recipes routes into single [recipeLang] structure to eliminate code duplication. All pages now use conditional API routing and reactive labels based on language parameter.

- Merge duplicate route structures into /[recipeLang] with 404 for invalid slugs
- Add English API endpoints for search, favorites, tags, and categories
- Implement language dropdown in header with localStorage persistence
- Convert all pages to use Svelte 5 runes (, , )
- Add German-only redirects (301) for add/edit pages
- Make all view pages (list, detail, filters, search, favorites) fully bilingual
- Remove floating language switcher in favor of header dropdown

src/routes/[recipeLang]/+layout.svelte

@@ -0,0 +1,34 @@
+<script>
+import Header from '$lib/components/Header.svelte'
+    import UserHeader from '$lib/components/UserHeader.svelte';
+let { data } = $props();
+
+let user = $derived(data.session?.user);
+
+const isEnglish = $derived(data.lang === 'en');
+const labels = $derived({
+	allRecipes: isEnglish ? 'All Recipes' : 'Alle Rezepte',
+	favorites: isEnglish ? 'Favorites' : 'Favoriten',
+	inSeason: isEnglish ? 'In Season' : 'In Saison',
+	category: isEnglish ? 'Category' : 'Kategorie',
+	icon: 'Icon',
+	keywords: isEnglish ? 'Keywords' : 'Stichwörter',
+	tips: isEnglish ? 'Tips' : 'Tipps'
+});
+</script>
+
+<Header>
+	<ul class=site_header slot=links>
+	<li><a href="/{data.recipeLang}">{labels.allRecipes}</a></li>
+	{#if user}
+		<li><a href="/{data.recipeLang}/favorites">{labels.favorites}</a></li>
+	{/if}
+	<li><a href="/{data.recipeLang}/season">{labels.inSeason}</a></li>
+	<li><a href="/{data.recipeLang}/category">{labels.category}</a></li>
+	<li><a href="/{data.recipeLang}/icon">{labels.icon}</a></li>
+	<li><a href="/{data.recipeLang}/tag">{labels.keywords}</a></li>
+	<li><a href="/rezepte/tips-and-tricks">{labels.tips}</a></li>
+	</ul>
+	<UserHeader slot=right_side {user} showLanguageSelector={true}></UserHeader>
+	<slot></slot>
+</Header>