feat(pwa): installierbar als App-Icon (Manifest + Icons)

Web-App-Manifest, App-Icons (192/512 + maskable, Rennmaus-Motiv) und
apple-touch-icon/theme-color in index.html → "Zum Startbildschirm hinzufügen".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

gerbil-manager-web/public/manifest.webmanifest

@@ -0,0 +1,17 @@
+{
+  "name": "Rennmaus-Manager",
+  "short_name": "Rennmäuse",
+  "description": "Verwaltung für Rennmäuse, Würfe und Züchter",
+  "lang": "de",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "portrait",
+  "background_color": "#F7EDDD",
+  "theme_color": "#B97B3F",
+  "icons": [
+    { "src": "/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "/icon-512.png", "sizes": "512x512", "type": "image/png" },
+    { "src": "/maskable-512.png", "sizes": "512x512", "type": "image/png", "purpose": "maskable" }
+  ]
+}