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

manifest.webmanifest

@@ -0,0 +1,30 @@
+{
+  "name": "Astrolabe - Vega-Lite Snippet Manager",
+  "short_name": "Astrolabe",
+  "description": "A lightweight, browser-based snippet manager for Vega-Lite visualizations",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#c0c0c0",
+  "theme_color": "#000080",
+  "orientation": "any",
+  "icons": [
+    {
+      "src": "src/favicon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any"
+    },
+    {
+      "src": "icon-192x192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any maskable"
+    },
+    {
+      "src": "icon-512x512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any maskable"
+    }
+  ]
+}