Initial SvelteKit frontend port of windwiderstand.de

Full parity with Astro site: content rows, post/tag routes, pagination,
event badges + OSM map, comments, Live-Search via /api/search-index,
CMS image proxy, RSS, sitemap.

Deploy: Dockerfile + docker-compose.prod.yml + Gitea Actions workflow
to build + push to git.pm86.de registry and ssh-deploy to Contabo.

src/lib/overlay-store.ts

@@ -0,0 +1,7 @@
+import { writable } from "svelte/store";
+
+/** true = Backdrop über Content anzeigen (Menü oder Suche offen). */
+export const overlayOpen = writable(false);
+
+/** Callback zum Schließen (setzt im Header menuOpen/searchOpen auf false). */
+export const overlayClose = writable<(() => void) | null>(null);