Add favicon

Small bingo-grid mark in the shared mono-green style used by sibling projects.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

client/public/favicon.svg

@@ -0,0 +1,11 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
+  <rect width="32" height="32" rx="7" fill="#080808"/>
+  <g fill="none" stroke="#00e87a" stroke-width="1.6">
+    <rect x="7" y="7" width="18" height="18" rx="1.5"/>
+    <line x1="13" y1="7" x2="13" y2="25"/>
+    <line x1="19" y1="7" x2="19" y2="25"/>
+    <line x1="7" y1="13" x2="25" y2="13"/>
+    <line x1="7" y1="19" x2="25" y2="19"/>
+  </g>
+  <circle cx="16" cy="16" r="2.1" fill="#00e87a"/>
+</svg>