feat(pwa): installierbar + Offline-Grundgeruest

- Manifest (name/standalone/theme-color wald), Icons aus echtem Logo
  (192/512/maskable/apple-touch) via scripts/generate-pwa-icons.mjs
- app.html: manifest + theme-color + apple-touch-icon + iOS-Meta
- service-worker.ts: precache Build-Assets, Seiten network-first mit
  Offline-Cache-Fallback (SvelteKit registriert autom. in Prod)
- Push (VAPID) folgt separat

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/service-worker.ts

@@ -0,0 +1,69 @@
+/// <reference types="@sveltejs/kit" />
+/// <reference no-default-lib="true"/>
+/// <reference lib="esnext" />
+/// <reference lib="webworker" />
+import { build, files, version } from "$service-worker";
+
+const sw = self as unknown as ServiceWorkerGlobalScope;
+
+const CACHE = `ww-cache-${version}`;
+// Vom Build erzeugte Assets + statische Dateien (Icons, Manifest, Fonts…).
+const PRECACHE = [...build, ...files];
+
+sw.addEventListener("install", (event) => {
+  event.waitUntil(
+    caches
+      .open(CACHE)
+      .then((cache) => cache.addAll(PRECACHE))
+      .then(() => sw.skipWaiting()),
+  );
+});
+
+sw.addEventListener("activate", (event) => {
+  event.waitUntil(
+    (async () => {
+      for (const key of await caches.keys()) {
+        if (key !== CACHE) await caches.delete(key);
+      }
+      await sw.clients.claim();
+    })(),
+  );
+});
+
+sw.addEventListener("fetch", (event) => {
+  const req = event.request;
+  if (req.method !== "GET") return;
+
+  const url = new URL(req.url);
+  // Nur same-origin — CMS-/Cross-Origin-Requests (dynamischer Content) nicht cachen.
+  if (url.origin !== sw.location.origin) return;
+
+  // Precache-Assets → cache-first (unveränderlich pro Version).
+  if (PRECACHE.includes(url.pathname)) {
+    event.respondWith(caches.match(req).then((r) => r ?? fetch(req)));
+    return;
+  }
+
+  // Seiten → network-first, bei Offline aus Cache; sonst Hinweis.
+  if (req.mode === "navigate") {
+    event.respondWith(
+      (async () => {
+        try {
+          const res = await fetch(req);
+          const cache = await caches.open(CACHE);
+          cache.put(req, res.clone());
+          return res;
+        } catch {
+          const cached = await caches.match(req);
+          return (
+            cached ??
+            new Response("Du bist offline. Diese Seite wurde noch nicht geladen.", {
+              status: 503,
+              headers: { "Content-Type": "text/plain; charset=utf-8" },
+            })
+          );
+        }
+      })(),
+    );
+  }
+});