feat(pwa): Cache-Strategy-Entscheider + Unit-Tests

Reine TS-Funktion resolveStrategy({url, method}) → 'shell' | 'swr'
| 'images' | 'network-only'. Kernregel:
  - Schreib-Methoden + import/preview/search/web → network-only
  - /images/* → images (cache-first)
  - /_app/, manifest, Icons, favicon, robots → shell (cache-first)
  - Alles andere same-origin-GET → swr
7 Tests decken alle Buckets ab. Wird vom SW in Task 8 aufgerufen.

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

src/lib/sw/cache-strategy.ts

@@ -0,0 +1,42 @@
+export type CacheStrategy = 'shell' | 'swr' | 'images' | 'network-only';
+
+export type RequestShape = { url: string; method: string };
+
+// Pure function — sole decision-maker for "which strategy for this request?".
+// Called by the service worker for every fetch event.
+export function resolveStrategy(req: RequestShape): CacheStrategy {
+  // All write methods: never cache.
+  if (req.method !== 'GET' && req.method !== 'HEAD') return 'network-only';
+
+  // Reduce URL to pathname — query string not needed for matching
+  // except that online-only endpoints need no special handling here.
+  const path = req.url.startsWith('http') ? new URL(req.url).pathname : req.url.split('?')[0];
+
+  // Explicitly online-only GETs
+  if (
+    path === '/api/recipes/import' ||
+    path === '/api/recipes/preview' ||
+    path.startsWith('/api/recipes/search/web')
+  ) {
+    return 'network-only';
+  }
+
+  // Images
+  if (path.startsWith('/images/')) return 'images';
+
+  // App-shell: build assets and known static files
+  if (
+    path.startsWith('/_app/') ||
+    path === '/manifest.webmanifest' ||
+    path === '/icon.svg' ||
+    path === '/icon-192.png' ||
+    path === '/icon-512.png' ||
+    path === '/favicon.ico' ||
+    path === '/robots.txt'
+  ) {
+    return 'shell';
+  }
+
+  // Everything else: recipe pages, API reads, lists — all SWR.
+  return 'swr';
+}