pwa: add app icon SVG and update manifest

Create a minimal SVG icon (512x512, dark rounded square with blue 'M')
and update manifest.json to reference it with purpose 'any'.

This fixes the blank tile on mobile add-to-home-screen and PWA install
prompts.

cmd/mavweb/static/manifest.json

@@ -5,6 +5,13 @@
   "display": "standalone",
   "background_color": "#111",
   "theme_color": "#00aaff",
-  "icons": [],
+  "icons": [
+    {
+      "src": "/icon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any"
+    }
+  ],
   "description": "Voice client and dashboard for Maven"
 }