refactor: Promote basepath to environment variable (#24445)

Co-authored-by: crazywoola <100913391+crazywoola@users.noreply.github.com>

web/utils/var-basePath.js

@@ -1,6 +1,6 @@
 // export basePath to next.config.js
 // same as the one exported from var.ts
 module.exports = {
-  basePath: '',
+  basePath: process.env.NEXT_PUBLIC_BASE_PATH || '',
   assetPrefix: '',
 }