fix(server): guard flash-attn install behind OS platform check

The win_amd64 wheel URL was attempted on any OS with matching Python/
torch/CUDA tags. On Linux CUDA setups with VIBEPOD_ENABLE_FLASH_ATTN=1
this caused `uv pip install` to fail with an incompatible wheel; with
set -e the script then exited before launch instead of falling back to
SDPA.

- Add uname -s case statement inside the version-tag match: only set
  the wheel URL on MINGW*/CYGWIN*/MSYS* (Windows/Git Bash); all other
  platforms print a clear message and leave FLASH_ATTN_WHEEL_URL empty
- Move the install step into a separate `if [[ -n "$FLASH_ATTN_WHEEL_URL" ]]`
  block so non-Windows platforms skip it entirely
- Wrap `uv pip install` in an `if` so a wheel failure is non-fatal and
  falls through to SDPA regardless of set -e
- Update header comment to reflect cross-platform behaviour

server/start.sh

@@ -9,8 +9,8 @@
 #
 # Optional CUDA acceleration:
 #   VIBEPOD_ENABLE_FLASH_ATTN=1 ./start.sh
-#   Installs a matching third-party Windows flash-attn wheel when the CUDA venv
-#   uses Python 3.12, torch 2.6.0, and CUDA 12.4.
+#   Installs a pre-built flash-attn wheel when the CUDA venv uses Python 3.12,
+#   torch 2.6.0, and CUDA 12.4 on Windows. Other platforms fall back to SDPA.
 #
 # The two modes maintain completely separate virtual environments so their torch
 # installations never conflict. UV_PROJECT_ENVIRONMENT tells uv which venv to use;
@@ -106,10 +106,25 @@ else
             TORCH_TAG="$(uv run python -c "import torch; print(torch.__version__.split('+', 1)[0])")"
             CUDA_TAG="$(uv run python -c "import torch; print('cu' + torch.version.cuda.replace('.', ''))")"
 
+            FLASH_ATTN_WHEEL_URL=""
             if [[ "$PY_TAG" == "cp312" && "$TORCH_TAG" == "2.6.0" && "$CUDA_TAG" == "cu124" ]]; then
+                case "$(uname -s)" in
+                    MINGW*|CYGWIN*|MSYS*)
                         FLASH_ATTN_WHEEL_URL="https://huggingface.co/lldacing/flash-attention-windows-wheel/resolve/main/flash_attn-2.7.4%2Bcu124torch2.6.0cxx11abiFALSE-cp312-cp312-win_amd64.whl"
-                echo "    Installing flash-attn for Python 3.12, torch 2.6.0, CUDA 12.4..."
-                uv pip install "$FLASH_ATTN_WHEEL_URL"
+                        echo "    Installing flash-attn for Python 3.12, torch 2.6.0, CUDA 12.4 (Windows)..."
+                        ;;
+                    *)
+                        echo "    No pre-built flash-attn wheel available for this platform ($(uname -s))."
+                        echo "    Continuing with PyTorch SDPA attention."
+                        ;;
+                esac
+            else
+                echo "    No known wheel for Python tag $PY_TAG, torch $TORCH_TAG, CUDA $CUDA_TAG."
+                echo "    Continuing with PyTorch SDPA attention."
+            fi
+
+            if [[ -n "$FLASH_ATTN_WHEEL_URL" ]]; then
+                if uv pip install "$FLASH_ATTN_WHEEL_URL"; then
                     if validate_flash_attn; then
                         echo "    flash-attn import check passed."
                     else
@@ -117,8 +132,8 @@ else
                         uv pip uninstall flash-attn
                     fi
                 else
-                echo "    No known wheel for Python tag $PY_TAG, torch $TORCH_TAG, CUDA $CUDA_TAG."
-                echo "    Continuing with PyTorch SDPA attention."
+                    echo "    flash-attn wheel install failed; continuing with SDPA."
+                fi
             fi
         fi
     fi