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/env.d.ts

@@ -0,0 +1,12 @@
+interface ImportMetaEnv {
+  readonly PUBLIC_WEB_URL: string
+  readonly PUBLIC_BASE_URL: string
+  readonly PUBLIC_ADMIN_URL: string
+  readonly PUBLIC_MAXKB_URL: string
+  readonly PUBLIC_ICONIFY_URL: string
+  readonly PUBLIC_WS_URL: string
+}
+
+interface ImportMeta {
+  readonly env: ImportMetaEnv
+}