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/icon-maskable.svg

@@ -0,0 +1,13 @@
+<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>
+  <!-- Maskable: Vollflächiger Hintergrund, Glyphe komplett in der 80%-Sicherheitszone -->
+  <rect width="512" height="512" fill="url(#g)"/>
+  <rect x="146" y="166" width="220" height="196" rx="36" fill="#0b0f1a" opacity="0.88"/>
+  <path d="M196 134v46M316 134v46" stroke="#ffffff" stroke-width="22" stroke-linecap="round" opacity="0.95"/>
+  <path d="M190 264l40 40 72-72" stroke="#ffffff" stroke-width="28" stroke-linecap="round" stroke-linejoin="round" fill="none"/>
+</svg>