feat: add Service Worker for offline resilience

sw.js

@@ -0,0 +1,68 @@
+const CACHE_NAME = 'the-door-v1';
+const ASSETS = [
+  '/',
+  '/index.html',
+  '/about'
+];
+
+// Install event - cache core assets
+self.addEventListener('install', (event) => {
+  event.waitUntil(
+    caches.open(CACHE_NAME).then((cache) => {
+      return cache.addAll(ASSETS);
+    })
+  );
+  self.skipWaiting();
+});
+
+// Activate event - cleanup old caches
+self.addEventListener('activate', (event) => {
+  event.waitUntil(
+    caches.keys().then((keys) => {
+      return Promise.all(
+        keys.filter((key) => key !== CACHE_NAME).map((key) => caches.delete(key))
+      );
+    })
+  );
+  self.clients.claim();
+});
+
+// Fetch event - network first, fallback to cache for static, 
+// but for the crisis front door, we want to ensure the shell is ALWAYS available.
+self.addEventListener('fetch', (event) => {
+  const url = new URL(event.request.url);
+
+  // Skip API calls - they should always go to network
+  if (url.pathname.startsWith('/api/')) {
+    return;
+  }
+
+  event.respondWith(
+    fetch(event.request)
+      .then((response) => {
+        // If we got a valid response, cache it for next time
+        if (response.ok && ASSETS.includes(url.pathname)) {
+          const copy = response.clone();
+          caches.open(CACHE_NAME).then((cache) => cache.put(event.request, copy));
+        }
+        return response;
+      })
+      .catch(() => {
+        // If network fails, try cache
+        return caches.match(event.request).then((cached) => {
+          if (cached) return cached;
+          
+          // If it's a navigation request and we're offline, return index.html
+          if (event.request.mode === 'navigate') {
+            return caches.match('/index.html');
+          }
+          
+          return new Response('Offline and resource not cached.', {
+            status: 503,
+            statusText: 'Service Unavailable',
+            headers: new Headers({ 'Content-Type': 'text/plain' })
+          });
+        });
+      })
+  );
+});