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>

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>

frontend/public/icons/icon.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>
+  <rect width="512" height="512" rx="112" 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>

frontend/public/manifest.webmanifest

@@ -0,0 +1,22 @@
+{
+  "name": "WannPassts",
+  "short_name": "WannPassts",
+  "description": "Deine freien Zeiten – geteilt per Link. Verbinde Google-, iCloud- und andere Kalender und nimm Buchungsanfragen an, ohne deine Termine preiszugeben.",
+  "lang": "de",
+  "dir": "ltr",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "any",
+  "background_color": "#05070c",
+  "theme_color": "#05070c",
+  "icons": [
+    { "src": "/icons/icon-192.png", "sizes": "192x192", "type": "image/png", "purpose": "any" },
+    { "src": "/icons/icon-512.png", "sizes": "512x512", "type": "image/png", "purpose": "any" },
+    { "src": "/icons/icon-maskable-512.png", "sizes": "512x512", "type": "image/png", "purpose": "maskable" }
+  ],
+  "shortcuts": [
+    { "name": "Anfragen", "url": "/app?tab=requests" },
+    { "name": "Kalender", "url": "/app?tab=calendars" }
+  ]
+}