Add ninja to requirements.txt

[andybiar]

I tried running the setup instructions on my local Mac, but got an import error:

ImportError: cannot import name 'csrc' from 'gsplat' (/Users/andy.biar/Projects/splat_analyzer/third_party/gsplat-mps/gsplat/init.py)

After consulting with my local Claude instance, Claude determined that this line was supposed to fail, so that backend.py could catch the issue and JIT-compile the MPS extension at runtime. However, ninja needs to be installed to get this working. Without ninja installed, we see this error following the previous error:

File "/Users/andy.biar/Projects/splat_analyzer/.venv/lib/python3.14/site-packages/torch/utils/cpp_extension.py", line 2451, in verify_ninja_availability
raise RuntimeError("Ninja is required to load C++ extensions (pip install ninja to get it)")
RuntimeError: Ninja is required to load C++ extensions (pip install ninja to get it)

After installing ninja, I'm able get splat analyzer running.