Add PWA and mobile creation flow

static/icon.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512">
+  <rect width="512" height="512" rx="110" fill="#212529"/>
+  <path d="M146 160h220M146 256h220M146 352h140" stroke="#fff" stroke-width="34" stroke-linecap="round"/>
+  <circle cx="104" cy="160" r="18" fill="#0dcaf0"/>
+  <circle cx="104" cy="256" r="18" fill="#ffc107"/>
+  <circle cx="104" cy="352" r="18" fill="#198754"/>
+</svg>

static/manifest.webmanifest

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+{
+  "name": "my2dos",
+  "short_name": "my2dos",
+  "description": "Notizen, Aufgaben, Links und Journal",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "background_color": "#f7f7f4",
+  "theme_color": "#212529",
+  "icons": [
+    {
+      "src": "/static/icon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any maskable"
+    }
+  ]
+}

static/sw.js

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+self.addEventListener('install', event => self.skipWaiting());
+self.addEventListener('activate', event => event.waitUntil(self.clients.claim()));
+
+// Minimaler Service Worker für Installierbarkeit als PWA.
+// Offline-Funktionalität ist bewusst nicht implementiert.
+self.addEventListener('fetch', () => {});