Add service worker with workbox: asset/image/font cache, offline fallback

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

config/packages/pwa.yaml

@@ -41,6 +41,19 @@ pwa:
               short_name: Contact
               url: /contact
               description: Nous contacter
+    serviceworker:
+        src: "sw.js"
+        workbox:
+            asset_cache:
+                enabled: true
+                regex: '/\.(css|js|json|xml|txt|map|ico|png|jpe?g|gif|svg|webp|bmp)$/'
+                max_entries: 60
+            image_cache:
+                enabled: true
+            font_cache:
+                enabled: true
+            offline_fallback:
+                page: 'app_offline_page'
     favicons:
         enabled: true
         default: