feat: PWA setup + /app base path

- Landing page at /
- Vue app at /app (PWA installable)
- Added manifest.json for mobile install
- iOS + Android meta tags
- Updated router base path

public/manifest.json

@@ -0,0 +1,25 @@
+{
+  "name": "SeCu - Mitarbeiterverwaltung",
+  "short_name": "SeCu",
+  "description": "Deine Aufträge, Stundenzettel und Dienstpläne",
+  "start_url": "/app/",
+  "scope": "/app/",
+  "display": "standalone",
+  "background_color": "#667eea",
+  "theme_color": "#667eea",
+  "orientation": "portrait-primary",
+  "icons": [
+    {
+      "src": "/app/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any maskable"
+    },
+    {
+      "src": "/app/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any maskable"
+    }
+  ]
+}