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",

public/sw.js

@@ -12,9 +12,9 @@
  *   API: Immer Netzwerk (kein Caching von Nutzerdaten)
  */
 
-const SHELL_CACHE   = 'oikos-shell-v20';
-const PAGES_CACHE   = 'oikos-pages-v20';
-const ASSETS_CACHE  = 'oikos-assets-v20';
+const SHELL_CACHE   = 'oikos-shell-v21';
+const PAGES_CACHE   = 'oikos-pages-v21';
+const ASSETS_CACHE  = 'oikos-assets-v21';
 const ALL_CACHES    = [SHELL_CACHE, PAGES_CACHE, ASSETS_CACHE];
 
 // App-Shell: sofort benötigt für ersten Render
@@ -23,7 +23,10 @@ const APP_SHELL = [
   '/index.html',
   '/api.js',
   '/router.js',
+  '/i18n.js',
   '/rrule-ui.js',
+  '/locales/de.json',
+  '/locales/en.json',
   '/sw-register.js',
   '/lucide.min.js',
   '/styles/tokens.css',