Replace pip & requirements.txt with uv & pyproject.toml

[spwoodcock]

Fixes #1936

Work in progress.
Built in top of #1942, so the commits will look a lot cleaner once merged.

Tasks:

• Migrate requirements.txt --> pyproject.toml (incl platform specific deps)
• Update context.py and other places that reference local installs of packages like opencv, opensfm
• Create a scripts section in pyproject.toml for CLI usage
• Update all references to pip / pip3 --> uv
• Update snapcraft builds (do we still maintain these? If so, I can look at them. If not, no point spending time)
• Tweak Dockerfiles to use UV
• Lots of testing for Linux / Windows (+ a community member to test the MacOS builds 🙏)