fix: use --index-url for NVIDIA torch install, pass --cuda-version to fast-deps path

[bigcat88]

Same class of bug as #379 (AMD ROCm fix), but for NVIDIA on Windows and Linux.

The NVIDIA pip path used --extra-index-url for all CUDA versions on Windows. When the CUDA index has an older torch than PyPI (cu118 tops at 2.7.1, cu121 at 2.5.1, cu124 at 2.6.0), pip picks PyPI's 2.10.0 which is CPU-only on Windows. Users selecting --nvidia --cuda-version 12.4 on Windows silently get a CPU torch.

On Linux, the NVIDIA path had no index flag at all — it just used plain PyPI. This meant --cuda-version was completely ignored.

The fix changes --extra-index-url to --index-url and makes the NVIDIA block platform-agnostic (same approach as the AMD fix in #379). The 5 repetitive elif blocks are collapsed into a single block that computes the CUDA tag dynamically from the --cuda-version enum value.

Additionally, the --fast-deps / uv path (DependencyCompiler) had hardcoded cu126 and rocm6.3 — it ignored both --cuda-version and --rocm-version. This PR adds cuda_version and rocm_version optional parameters to DependencyCompiler.init and passes them through from execute().

Verified with real pip --dry-run and real uv pip compile against all 10 index URLs (5 CUDA + 5 ROCm). All resolve to the correct GPU-specific torch. Confirmed the old --extra-index-url behavior was broken for cu118, cu121, cu124, rocm6.1, rocm6.2, rocm6.3 (PyPI's CPU torch won in all cases).

Relates to #289.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main     #380      +/-   ##
==========================================
+ Coverage   63.10%   63.44%   +0.34%     
==========================================
  Files          33       33              
  Lines        3697     3699       +2     
==========================================
+ Hits         2333     2347      +14     
+ Misses       1364     1352      -12     

Files with missing lines	Coverage Δ
comfy_cli/command/install.py	72.74% <100.00%> (+1.87%)	⬆️
comfy_cli/uv.py	57.66% <100.00%> (+1.77%)	⬆️

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