feat: add Progressive Web App support with service worker and offline functionality

sw.js

@@ -0,0 +1,82 @@
+const CACHE_NAME = 'astrolabe-v1.0.0';
+const URLS_TO_CACHE = [
+  '/',
+  '/index.html',
+  '/src/styles.css',
+  '/src/favicon.svg',
+  '/src/js/config.js',
+  '/src/js/snippet-manager.js',
+  '/src/js/dataset-manager.js',
+  '/src/js/chart-builder.js',
+  '/src/js/panel-manager.js',
+  '/src/js/editor.js',
+  '/src/js/user-settings.js',
+  '/src/js/generic-storage-ui.js',
+  '/src/js/app.js'
+];
+
+// CDN URLs to cache
+const CDN_URLS = [
+  'https://cdnjs.cloudflare.com/ajax/libs/monaco-editor/0.47.0/min/vs/loader.js',
+  'https://unpkg.com/vega@5/build/vega.min.js',
+  'https://unpkg.com/vega-lite@5/build/vega-lite.min.js',
+  'https://unpkg.com/vega-embed@6/build/vega-embed.min.js',
+  'https://fonts.googleapis.com/css2?family=IBM+Plex+Mono:wght@400;500;600&display=swap'
+];
+
+// Install event - cache all static assets
+self.addEventListener('install', (event) => {
+  event.waitUntil(
+    caches.open(CACHE_NAME).then((cache) => {
+      // Cache local files
+      const localCachePromise = cache.addAll(URLS_TO_CACHE);
+
+      // Cache CDN files - they'll be cached during runtime via fetch event
+      // This avoids CORS issues during install phase
+      return localCachePromise;
+    })
+  );
+  // Force the waiting service worker to become active
+  self.skipWaiting();
+});
+
+// Activate event - clean up old caches
+self.addEventListener('activate', (event) => {
+  event.waitUntil(
+    caches.keys().then((cacheNames) => {
+      return Promise.all(
+        cacheNames.map((cacheName) => {
+          if (cacheName !== CACHE_NAME) {
+            return caches.delete(cacheName);
+          }
+        })
+      );
+    })
+  );
+  // Take control of all pages immediately
+  return self.clients.claim();
+});
+
+// Fetch event - serve from cache, fallback to network
+self.addEventListener('fetch', (event) => {
+  event.respondWith(
+    caches.match(event.request).then((response) => {
+      // Return cached version or fetch from network
+      return response || fetch(event.request).then((fetchResponse) => {
+        // Cache successful responses for future use
+        if (fetchResponse && fetchResponse.status === 200) {
+          const responseToCache = fetchResponse.clone();
+          caches.open(CACHE_NAME).then((cache) => {
+            cache.put(event.request, responseToCache);
+          });
+        }
+        return fetchResponse;
+      });
+    }).catch(() => {
+      // Offline fallback - return cached index for navigation requests
+      if (event.request.mode === 'navigate') {
+        return caches.match('/index.html');
+      }
+    })
+  );
+});