Support non-git installations

### Feature Idea

**Is your feature request related to a problem?**
When ComfyUI is installed from a tarball/release archive (not git clone), the Manager shows "Your ComfyUI isn't git repo" and update functionality is broken.

Example - Community-Scripts ProxmoxVE:
https://github.com/community-scripts/ProxmoxVE/issues/9738

Installer:
https://raw.githubusercontent.com/community-scripts/ProxmoxVE/main/install/comfyui-install.sh

Possible Solution:
Support updating ComfyUI installations that aren't git repositories by:
1. Downloading the latest release tarball
2. Extracting and replacing files (preserving user data like custom_nodes, models, etc.)

**Why is this useful?**
- Many automated deployment scripts (like our Proxmox community-scripts) use tarballs for reproducible, versioned installs
- Tarballs are smaller and faster to download than full git clones
- Users shouldn't need git installed just to update

### Existing Solutions

Manual download and extraction of new releases, or converting to git repo post-install.

### Other

_No response_