Configure uv to always use PyTorch CUDA index

Changes:
- Set PyTorch CUDA index (cu121) as default for all builds
- CUDA builds support both GPU and CPU (auto-fallback)
- Fixes uv run reinstalling CPU-only PyTorch
- Updated dependency-groups syntax (fixes deprecation warning)

Benefits:
- Simpler build process - no CPU vs CUDA distinction needed
- uv sync and uv run now get CUDA-enabled PyTorch automatically
- Builds work on systems with or without NVIDIA GPUs
- Fixes issue where uv run check_cuda.py was getting CPU version

Index: https://download.pytorch.org/whl/cu121 (PyTorch 2.5.1+cu121)

pyproject.toml

@@ -52,11 +52,17 @@ build-backend = "hatchling.build"
 [tool.hatch.build.targets.wheel]
 packages = ["client", "gui"]
 
-[tool.uv]
-dev-dependencies = [
+[dependency-groups]
+dev = [
     "pyinstaller>=6.17.0",
 ]
 
+# Use PyTorch CUDA index by default
+# CUDA builds work on both GPU and CPU systems (fallback to CPU if no GPU)
+[[tool.uv.index]]
+url = "https://download.pytorch.org/whl/cu121"
+default = true
+
 [tool.ruff]
 line-length = 100
 target-version = "py39"