Extension install from GitHub fails for repos whose default branch is master (installer assumes main)

[jjfeiler]

Describe the bug

Installing an extension via Models → Install from GitHub fails with "no manifest.json or generator.py" when the extension repo's default branch is master. The installer appears to fetch the main branch archive unconditionally instead of resolving the repo's actual default branch, so the fetch 404s (or yields nothing) even though the required files exist at the repo root.

To Reproduce

Models → Install from GitHub
Paste https://github.com/DrHepa/modly-trellis-text-extension (default branch: master; has manifest.json and generator.py at root)
Install fails reporting missing manifest.json / generator.py

Steps to reproduce

Models → Install from GitHub
Paste https://github.com/DrHepa/modly-trellis-text-extension (default branch: master; has manifest.json and generator.py at root)
Install fails reporting missing manifest.json / generator.py

Modly version

0.4.1

Operating system

macOS 26.5.2

Screenshots & logs

No response

Confirmation

• I have searched existing issues and this has not been reported yet.
• I can reproduce this on the latest version of Modly.