Update model selection options in Main.py

Update Gemini API model to version 3.0

Update contribution link in README.md

Default to Gemini as it has a more generous free tier

Update model selection for OpenAI and Gemini

Switch to Gemini 2.0 Flash (2.5 is only in public preview for the API)

Re-introduce Alta theme (#35)

Streamlit Community Cloud works

Less hard-coded way of finding tectonic

Yet another attempt to make Streamlit cloud work

Include executable

Special handling for Streamlit Cloud

Try to find Tectonic in Streamlit Cloud

Fix for Streamlit Community Cloud: find LaTeX engine

Move deps to PyPI to make it easier for Streamlit Community Cloud to resolve