feat: v0.8.3 — PWA-Support, Desktop-Verhalten, Testmodus-Kennzeichnung

- PWA: manifest.webmanifest + Icons (192/512/180px) + Apple-Touch-Meta-Tags
  → Web-App lässt sich auf Homebildschirm hinzufügen (iOS/Android)
- Desktop: user-select:none global + contextmenu blockiert
- BackendChoice: lokaler Modus als Testmodus mit TEST-Badge und Warnung

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

public/manifest.webmanifest

@@ -0,0 +1,27 @@
+{
+  "name": "Rapport",
+  "short_name": "Rapport",
+  "description": "Studio-Management für Architekturbüros",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "portrait",
+  "background_color": "#ebe7e1",
+  "theme_color": "#1a1a18",
+  "lang": "de-CH",
+  "categories": ["business", "productivity"],
+  "icons": [
+    {
+      "src": "/pwa-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any maskable"
+    },
+    {
+      "src": "/pwa-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any maskable"
+    }
+  ]
+}