add web app manifest so site can be installed as a desktop app (v4)

Adds manifest.json + a 512x512 icon and links them from layout.njk, so
Edge/Chrome can install the site as a standalone app window from a
desktop shortcut while it keeps loading live from the server.

public/manifest.json

@@ -0,0 +1,12 @@
+{
+  "name": "Harding's Service",
+  "short_name": "Harding's Service",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#F2E8D5",
+  "theme_color": "#4A2810",
+  "icons": [
+    { "src": "/icons/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "/icons/icon-512.png", "sizes": "512x512", "type": "image/png" }
+  ]
+}