Windows compatibility issues in cp-css.js and openui-cli build:templates script

[Prakash21singh]

Describe the bug
Two issues cause the project setup to fail on Windows.

1. cp-css.js uses new URL(import.meta.url).pathname which produces an invalid path on Windows/Git Bash (e.g. \C:\Users...), crashing on directory creation.
2. The build:templates script in openui-cli uses Unix commands (rm -rf, mkdir -p, cp -R) which fail because pnpm defaults to CMD on Windows.

To Reproduce
Steps to reproduce the behavior:

1. Clone the repo on a Windows machine
2. Run pnpm install
3. See error: The syntax of the command is incorrect in openui-cli build
4. After working around that, run node cp-css.js from packages/react-ui
5. See error: ENOENT: no such file or directory, mkdir 'C:\C:\Users...'

Expected behavior
pnpm install and node cp-css.js should complete successfully on Windows without any manual workarounds.

Desktop (please complete the following information):

• OS: Windows 11
• Shell: Git Bash (MINGW64)
• Node : v22.20.0
• pnpm: 10.18.3

Additional context
Happy to raise a PR with fixes for both issues if the maintainers are open to it!