Fix Windows CI: use powershell instead of bash for uv steps

Windows runner doesn't have bash. Split Python setup and build steps
into Unix (default shell) and Windows (powershell) variants.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -54,12 +54,23 @@ jobs:
             echo "$env:USERPROFILE\.local\bin" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
           }
 
-      - name: Set up Python
-        shell: bash
+      - name: Set up Python (Unix)
+        if: matrix.platform != 'windows'
         run: uv python install ${{ env.PYTHON_VERSION }}
 
-      - name: Build sidecar
-        shell: bash
+      - name: Set up Python (Windows)
+        if: matrix.platform == 'windows'
+        shell: powershell
+        run: uv python install ${{ env.PYTHON_VERSION }}
+
+      - name: Build sidecar (Unix)
+        if: matrix.platform != 'windows'
+        working-directory: python
+        run: uv run --python ${{ env.PYTHON_VERSION }} python build_sidecar.py --cpu-only
+
+      - name: Build sidecar (Windows)
+        if: matrix.platform == 'windows'
+        shell: powershell
         working-directory: python
         run: uv run --python ${{ env.PYTHON_VERSION }} python build_sidecar.py --cpu-only