fix: improve PWA manifest and SW cache for Android standalone mode

- Add id field and display_override to manifest.json for reliable
  Chrome Android PWA recognition
- Serve manifest.json with application/manifest+json MIME type
- Add /i18n.js and locale files to SW app shell cache (were missing)
- Bump SW cache version to v21

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/manifest.json

@@ -2,9 +2,11 @@
   "name": "Oikos Familienplaner",
   "short_name": "Oikos",
   "description": "Selbstgehosteter Familienplaner",
+  "id": "/",
   "start_url": "/",
   "scope": "/",
   "display": "standalone",
+  "display_override": ["standalone", "minimal-ui"],
   "orientation": "portrait-primary",
   "theme_color": "#007AFF",
   "background_color": "#F5F5F7",