Fix uv install for Windows (use PowerShell installer)

Unix runners use the bash install script, Windows uses the PowerShell
installer. Both check if uv is already present first.

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

.gitea/workflows/build.yml

@@ -32,7 +32,8 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Install uv
+      - name: Install uv (Unix)
+        if: matrix.platform != 'windows'
         shell: bash
         run: |
           if command -v uv &> /dev/null; then
@@ -42,6 +43,17 @@ jobs:
             echo "$HOME/.local/bin" >> $GITHUB_PATH
           fi
 
+      - name: Install uv (Windows)
+        if: matrix.platform == 'windows'
+        shell: powershell
+        run: |
+          if (Get-Command uv -ErrorAction SilentlyContinue) {
+            Write-Host "uv already installed: $(uv --version)"
+          } else {
+            irm https://astral.sh/uv/install.ps1 | iex
+            echo "$env:USERPROFILE\.local\bin" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
+          }
+
       - name: Set up Python
         run: uv python install ${{ env.PYTHON_VERSION }}