fix(frontend): match favicon to logo (white cloud on blue gradient)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/app/icon.svg

@@ -0,0 +1,12 @@
+<svg width="512" height="512" viewBox="0 0 512 512" xmlns="http://www.w3.org/2000/svg">
+  <defs>
+    <linearGradient id="g" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0%" stop-color="#3b82f6" />
+      <stop offset="100%" stop-color="#1d4ed8" />
+    </linearGradient>
+  </defs>
+  <rect x="0" y="0" width="512" height="512" rx="128" fill="url(#g)" />
+  <g transform="translate(112 112) scale(12)" fill="none" stroke="#ffffff" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+    <path d="M17.5 19H9a7 7 0 1 1 6.71-9h1.79a4.5 4.5 0 1 1 0 9Z" />
+  </g>
+</svg>