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/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" }
+  ]
+}