feat: add PWA support and mobile-responsive layouts

- Add web app manifest, maskable/apple-touch icons, and a production-only
  service worker that caches the app shell for offline start
- Improve mobile UX: responsive breakpoints from ~320px, 44px touch
  targets, safe-area insets, dvh heights, 16px inputs to avoid iOS zoom
- Serve .webmanifest with correct content type in the Go static handler
- Document PWA behavior and mobile features in the README

frontend/public/icons/apple-touch-icon.svg

@@ -0,0 +1,14 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512">
+  <defs>
+    <linearGradient id="g" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0" stop-color="#7db0ff"/>
+      <stop offset="1" stop-color="#b78cff"/>
+    </linearGradient>
+  </defs>
+  <!-- Apple Touch Icon: vollflächig, ohne Transparenz (iOS maskiert selbst) -->
+  <rect width="512" height="512" fill="url(#g)"/>
+  <rect x="112" y="132" width="288" height="268" rx="46" fill="#0b0f1a" opacity="0.88"/>
+  <path d="M186 104v52M326 104v52" stroke="#ffffff" stroke-width="26" stroke-linecap="round" opacity="0.95"/>
+  <path d="M118 198h276" stroke="#ffffff" stroke-width="10" stroke-linecap="round" opacity="0.22"/>
+  <path d="M186 272l52 52 92-92" stroke="#ffffff" stroke-width="34" stroke-linecap="round" stroke-linejoin="round" fill="none"/>
+</svg>