feat: PWA manifest, app icons, service worker with tests

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

public/manifest.webmanifest

@@ -0,0 +1,16 @@
+{
+  "name": "Weerscherm",
+  "short_name": "Weer",
+  "description": "Netatmo-weerstation en verwachting voor in de woonkamer",
+  "start_url": "./",
+  "scope": "./",
+  "display": "standalone",
+  "orientation": "landscape",
+  "background_color": "#E8EDF1",
+  "theme_color": "#E8EDF1",
+  "lang": "nl",
+  "icons": [
+    { "src": "icons/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "icons/icon-512.png", "sizes": "512x512", "type": "image/png" }
+  ]
+}