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/icon.svg

@@ -0,0 +1,10 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512">
+  <defs>
+    <linearGradient id="bg" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0%" stop-color="#1a1a2e"/>
+      <stop offset="100%" stop-color="#0d0d1a"/>
+    </linearGradient>
+  </defs>
+  <rect width="512" height="512" rx="96" fill="url(#bg)"/>
+  <text x="256" y="340" font-family="system-ui,-apple-system,sans-serif" font-size="320" font-weight="700" fill="#00aaff" text-anchor="middle">M</text>
+</svg>

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"
 }