chore: 将前端移动到 client/ 目录，准备合并为单仓库

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

client/src/global.css

@@ -0,0 +1,27 @@
+* {
+  box-sizing: border-box;
+}
+
+body {
+  margin: 0;
+  -webkit-font-smoothing: antialiased;
+  -moz-osx-font-smoothing: grayscale;
+}
+
+::-webkit-scrollbar {
+  width: 6px;
+  height: 6px;
+}
+
+::-webkit-scrollbar-track {
+  background: transparent;
+}
+
+::-webkit-scrollbar-thumb {
+  background: #d0d0d0;
+  border-radius: 3px;
+}
+
+::-webkit-scrollbar-thumb:hover {
+  background: #b0b0b0;
+}