Add web app manifest and icons for PWA support

- Created a new site.webmanifest file to define the web app's metadata.
- Added icons for the web app in both 192x192 and 512x512 sizes.
- Set theme color and background color to #1171e9.
- Configured the display mode to standalone.

public/site.webmanifest

@@ -0,0 +1,21 @@
+{
+  "name": "cmzi.uk",
+  "short_name": "cmzi.uk",
+  "icons": [
+    {
+      "src": "/web-app-manifest-192x192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "maskable"
+    },
+    {
+      "src": "/web-app-manifest-512x512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
+    }
+  ],
+  "theme_color": "#1171e9",
+  "background_color": "#1171e9",
+  "display": "standalone"
+}