feat(pwa): offer immediate mobile Web-App installation sheet and header install button

frontend/public/icon.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512" width="512" height="512">
+  <rect width="512" height="512" rx="128" fill="#0b0f19"/>
+  <circle cx="256" cy="256" r="164" fill="none" stroke="#8ab4f8" stroke-width="36"/>
+  <path d="M184 266 L236 318 L338 194" fill="none" stroke="#c58af9" stroke-width="40" stroke-linecap="round" stroke-linejoin="round"/>
+</svg>

frontend/public/manifest.json

@@ -1,18 +1,26 @@
 {
+  "id": "/",
   "name": "MindSift — Der Kopf frei in drei Sekunden",
   "short_name": "MindSift",
   "description": "Instant Brain-Dump, Tinder-Style Triage, Workspace & Echtzeit-Familienlisten.",
-  "start_url": "/?mode=capture",
+  "start_url": "/",
+  "scope": "/",
   "display": "standalone",
   "background_color": "#0b0f19",
   "theme_color": "#0b0f19",
   "orientation": "portrait-primary",
   "icons": [
     {
-      "src": "data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'><rect width='100' height='100' rx='26' fill='%23131314'/><circle cx='50' cy='50' r='32' fill='none' stroke='%238ab4f8' stroke-width='7'/><path d='M36 52 L46 62 L66 38' fill='none' stroke='%23c58af9' stroke-width='8' stroke-linecap='round' stroke-linejoin='round'/></svg>",
+      "src": "/icon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any"
+    },
+    {
+      "src": "/icon.svg",
       "sizes": "192x192 512x512",
       "type": "image/svg+xml",
-      "purpose": "any maskable"
+      "purpose": "maskable"
     }
   ],
   "shortcuts": [