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/favicon.svg

@@ -0,0 +1,10 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
+  <defs>
+    <linearGradient id="grad" x1="0%" y1="0%" x2="100%" y2="100%">
+      <stop offset="0%" style="stop-color:#667eea"/>
+      <stop offset="100%" style="stop-color:#764ba2"/>
+    </linearGradient>
+  </defs>
+  <rect width="100" height="100" rx="20" fill="url(#grad)"/>
+  <text x="50" y="68" font-size="50" text-anchor="middle" fill="white">🔐</text>
+</svg>

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"
+    }
+  ]
+}