Add AMD ROCm GPU support

Introduces a third hardware mode alongside CUDA and CPU: ROCm (AMD GPU).
AMD GPUs present as CUDA devices under PyTorch ROCm, so the existing GPU
path is reused with minimal changes — the main additions are wheel management,
device detection, and suppressing flash_attn (unsupported on ROCm).

- server/vibevoice_server.py: extend _resolve_device() to recognise 'rocm'
  (auto-detected via torch.version.hip); add _torch_device() helper that maps
  'rocm' → 'cuda' for all PyTorch API calls; apply GPU optimisations for both
  cuda and rocm in _init_model(); always use sdpa on ROCm; propagate
  _torch_device() to _load_voice_presets() map_location.
- server/start.sh: add --rocm flag; sync .venv-rocm with uv sync --no-sources
  then replace torch with the ROCm 6.2 wheel via uv pip install; set
  VIBEPOD_DEVICE=rocm for uvicorn.
- server/pyproject.toml: register pytorch-rocm62 index (explicit); add
  .venv-rocm to ruff excludes.
- package.json: add dev:rocm and dev:server:rocm scripts.
- README.md: document ROCm mode, prerequisites (RX 6000+, ROCm 6.2+, Linux),
  and new commands; expand CUDA vs CPU section to CUDA vs CPU vs ROCm.

https://claude.ai/code/session_0168pSswiaoEf6LEx6UQWfBu

server/pyproject.toml

@@ -32,7 +32,7 @@ dev = [
 line-length = 100
 indent-width = 4
 target-version = "py310"
-exclude = [".git", ".venv", ".venv-cpu", "__pycache__"]
+exclude = [".git", ".venv", ".venv-cpu", ".venv-rocm", "__pycache__"]
 
 [tool.ruff.lint]
 select = ["E", "F", "UP", "B", "SIM", "I"]
@@ -54,6 +54,11 @@ name = "pytorch-cu124"
 url = "https://download.pytorch.org/whl/cu124"
 explicit = true
 
+[[tool.uv.index]]
+name = "pytorch-rocm62"
+url = "https://download.pytorch.org/whl/rocm6.2"
+explicit = true
+
 [tool.uv.sources]
 # Pull torch from the PyTorch CUDA 12.4 index instead of PyPI's CPU-only wheel.
 # CUDA 12.4 runs on any driver >= 525.60 (RTX 30/40 series all qualify).