Add PWA support (manifest, service worker, icons, safe-area CSS)

app/public/sw.js

@@ -0,0 +1,66 @@
+'use strict';
+
+const CACHE = 'weather-v1';
+
+// App shell — cached on install for offline use
+const PRECACHE = [
+  '/',
+  '/app.js',
+  '/style.css',
+  '/manifest.json',
+  '/icons/icon.svg'
+];
+
+self.addEventListener('install', event => {
+  event.waitUntil(
+    caches.open(CACHE)
+      .then(cache => cache.addAll(PRECACHE))
+      .then(() => self.skipWaiting())
+  );
+});
+
+self.addEventListener('activate', event => {
+  event.waitUntil(
+    caches.keys()
+      .then(keys => Promise.all(
+        keys.filter(k => k !== CACHE).map(k => caches.delete(k))
+      ))
+      .then(() => self.clients.claim())
+  );
+});
+
+self.addEventListener('fetch', event => {
+  const { request } = event;
+  const url = new URL(request.url);
+
+  // Skip non-GET, API calls, and map/radar tile requests
+  if (
+    request.method !== 'GET' ||
+    url.pathname.startsWith('/api/') ||
+    url.hostname.includes('rainviewer') ||
+    url.hostname.includes('openstreetmap') ||
+    url.hostname.includes('cartodb') ||
+    url.hostname.includes('basemaps')
+  ) {
+    return;
+  }
+
+  event.respondWith(
+    caches.match(request).then(cached => {
+      if (cached) return cached;
+      return fetch(request).then(response => {
+        // Only cache successful same-origin and trusted CDN responses
+        if (response.ok && (
+          url.origin === self.location.origin ||
+          url.hostname.includes('googleapis') ||
+          url.hostname.includes('gstatic') ||
+          url.hostname.includes('unpkg.com')
+        )) {
+          const clone = response.clone();
+          caches.open(CACHE).then(cache => cache.put(request, clone));
+        }
+        return response;
+      });
+    })
+  );
+});