Fallback for CLI arg --front-end-version

### Feature Idea

### Current

The `--front-end-version` command always connects to GitHub and downloads the frontend. If there is a connection error or GitHub is offline, it falls back to `web` - the stable frontend version that shipped with core.

### Proposed

a) Using `@latest`: Best-effort attempt to load the latest cached version from `web_custom_versions`.
b) Using specific version e.g. `@1.2.3`: Check for the exact cached version in `web_custom_versions`, load if present.

If no matching directory is found, fall back to `web` as normal.

### Existing Solutions

`--front-end-root` requires the user explicitly set the directory in the command.

### Other

_No response_