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]/+page.server.ts

@@ -0,0 +1,25 @@
+import type { PageServerLoad } from "./$types";
+import { getUserFavorites, addFavoriteStatusToRecipes } from "$lib/server/favorites";
+
+export const load: PageServerLoad = async ({ fetch, locals, params }) => {
+    const isEnglish = params.recipeLang === 'recipes';
+    const apiBase = isEnglish ? '/api/recipes' : '/api/rezepte';
+
+    let current_month = new Date().getMonth() + 1
+    const res_season = await fetch(`${apiBase}/items/in_season/` + current_month);
+    const res_all_brief = await fetch(`${apiBase}/items/all_brief`);
+    const item_season = await res_season.json();
+    const item_all_brief = await res_all_brief.json();
+
+    // Get user favorites and session
+    const [userFavorites, session] = await Promise.all([
+        getUserFavorites(fetch, locals),
+        locals.auth()
+    ]);
+
+    return {
+        season: addFavoriteStatusToRecipes(item_season, userFavorites),
+        all_brief: addFavoriteStatusToRecipes(item_all_brief, userFavorites),
+        session
+    };
+};