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

frontend/index.html

@@ -4,12 +4,23 @@
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no" />
     <meta name="theme-color" content="#0b0f19" />
+    <meta name="mobile-web-app-capable" content="yes" />
     <meta name="apple-mobile-web-app-capable" content="yes" />
+    <meta name="apple-mobile-web-app-title" content="MindSift" />
     <meta name="apple-mobile-web-app-status-bar-style" content="black-translucent" />
     <meta name="description" content="MindSift — Der Kopf frei in drei Sekunden. Instant Capture, Tinder-Style Triage, Workspace & Echtzeit-Listen." />
     <link rel="manifest" href="/manifest.json" />
-    <link rel="icon" type="image/svg+xml" href="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>" />
+    <link rel="icon" type="image/svg+xml" href="/icon.svg" />
+    <link rel="apple-touch-icon" href="/icon.svg" />
     <title>MindSift — Der Kopf frei in drei Sekunden</title>
+    <script>
+      window.__deferredPwaPrompt = null;
+      window.addEventListener('beforeinstallprompt', function (e) {
+        e.preventDefault();
+        window.__deferredPwaPrompt = e;
+        window.dispatchEvent(new CustomEvent('mindsift:pwa-ready'));
+      });
+    </script>
   </head>
   <body class="bg-[#0b0f19] text-[#e3e3e3] antialiased selection:bg-[#8ab4f8]/30 selection:text-[#8ab4f8]">
     <div id="root"></div>
@@ -19,9 +30,9 @@
         caches.keys().then((keys) => keys.forEach((k) => caches.delete(k))).catch(() => {});
       }
       if ('serviceWorker' in navigator) {
-        navigator.serviceWorker.getRegistrations().then((regs) => {
-          regs.forEach((r) => r.unregister());
-        }).catch(() => {});
+        window.addEventListener('load', () => {
+          navigator.serviceWorker.register('/sw.js').catch(() => {});
+        });
       }
     </script>
   </body>