feat(pwa): Service-Worker-Gerüst mit Shell-Cache + Fetch-Dispatch

src/service-worker.ts installiert die App-Shell-Assets (build +
files aus $service-worker) beim install-Event in kochwas-shell-
<version>, räumt alte Shell-Caches beim activate und dispatcht
jeden Fetch via resolveStrategy — shell/images cache-first, swr
stale-while-revalidate, network-only unangetastet. Pre-Cache-
Orchestrator kommt in Task 9.

Client-seitig registriert sw-register.ts den SW und verdrahtet
Messages vom SW in den sync-status-Store.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/routes/+layout.svelte

@@ -15,6 +15,7 @@
   import Toast from '$lib/components/Toast.svelte';
   import SyncIndicator from '$lib/components/SyncIndicator.svelte';
   import { network } from '$lib/client/network.svelte';
+  import { registerServiceWorker } from '$lib/client/sw-register';
   import type { SearchHit } from '$lib/server/recipes/search-local';
   import type { WebHit } from '$lib/server/search/searxng';
 
@@ -206,6 +207,7 @@
     void searchFilterStore.load();
     void pwaStore.init();
     network.init();
+    void registerServiceWorker();
     document.addEventListener('click', handleClickOutside);
     document.addEventListener('keydown', handleKey);
     return () => {