pnpm output loses colors when `create-vinext-app` initializing the project

## Summary

When installing dependencies for the new project with `create-vinext-app`, the output printed by pnpm loses its ANSI colors and is displayed as plain text.

The same pnpm commands normally produce colored output when run directly in a terminal, so the CLI appears to be preventing or stripping color output when spawning pnpm.

## Reproduction

Start a new project with `create-vinext-app`

```bash
pnpm create vinext-app@latest test-app
```

Choose the project configurations and install the project dependencies, then you can see the pnpm output just loses its colors.

## Expected behavior

## Possible root cause

pnpm dependencies installing process is spawned by `installDeps()` with `exec()` function, and the `stdio` is set to `inherit`.

https://github.com/cloudflare/vinext/blob/056cd026b3a2fa12d67c9d2ca941e75d2a47670e/packages/vinext/src/init.ts#L356-L360

However, `exec()` accepts the parameter `stdio` and just doesn't pass it to the `spawn()`. The `stdio` is actually hard-coded as `["inherit", "pipe", "pipe"]`.

https://github.com/cloudflare/vinext/blob/056cd026b3a2fa12d67c9d2ca941e75d2a47670e/packages/vinext/src/init.ts#L465-L473

## Notes

The hard-coded `pipe` is for detecting approve builds error (related #2279). So fixing this issue can be quite tricky.

Start with packages/vinext/src/init.ts, reading installDeps() around lines 356-360 and exec() around lines 465-473, then reproduce with pnpm create vinext-app@latest test-app. Trace how stdio reaches the spawned pnpm process and how approve-build errors from #2279 are detected. Done means the initializer preserves pnpm's terminal colors without breaking that error detection.

Written by the indexing model from the issue text.