feat(web): add mobile-first read-only dashboard frontend

reaction_board/web/static/favicon.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64">
+  <defs><linearGradient id="g" x1="0" y1="0" x2="1" y2="1">
+    <stop offset="0" stop-color="#fb7299"/><stop offset="1" stop-color="#ff9fbc"/>
+  </linearGradient></defs>
+  <rect width="64" height="64" rx="18" fill="url(#g)"/>
+  <text x="32" y="42" font-size="34" font-weight="700" text-anchor="middle"
+    fill="#fff" font-family="system-ui, sans-serif">咻</text>
+</svg>