fix: set UTF-8 encoding in Windows Docker containers

Add PYTHONIOENCODING=utf-8 env var to all Windows Docker run commands,
and set console/output encoding to UTF-8 in the build script. Fixes
UnicodeEncodeError for characters like ≈ (\u2248) that the default
Windows code page (cp1252) cannot encode.

.github/workflows/book-build-container.yml

@@ -543,6 +543,7 @@ jobs:
 
           docker run --rm `
             -e PYTHONPATH=C:\workspace `
+            -e PYTHONIOENCODING=utf-8 `
             -v "$($PWD.Path):C:\workspace" `
             -w "C:\workspace\book\quarto" `
             ${{ env.CONTAINER_IMAGE }} `
@@ -576,6 +577,9 @@ jobs:
           # Write build script to a file to avoid pipe/stdin issues
           $scriptContent = @'
           $ErrorActionPreference = 'Stop'
+          [Console]::OutputEncoding = [System.Text.Encoding]::UTF8
+          $OutputEncoding = [System.Text.Encoding]::UTF8
+          chcp 65001 | Out-Null
           Write-Host "📋 Working directory: $(Get-Location)"
           Write-Host "📋 Listing config files:"
           Get-ChildItem config\_quarto-*.yml | ForEach-Object { Write-Host "  $_" }
@@ -607,6 +611,7 @@ jobs:
 
           docker run --rm `
             -e PYTHONPATH=C:\workspace `
+            -e PYTHONIOENCODING=utf-8 `
             -v "$($PWD.Path):C:\workspace" `
             -w "C:\workspace\book\quarto" `
             ${{ env.CONTAINER_IMAGE }} `
@@ -692,6 +697,7 @@ jobs:
 
           docker run --rm `
             -e PYTHONPATH=C:\workspace `
+            -e PYTHONIOENCODING=utf-8 `
             -v "$($PWD.Path):C:\workspace" `
             -w "C:\workspace\book\quarto\${{ matrix.output_dir }}" `
             ${{ env.CONTAINER_IMAGE }} `
@@ -788,6 +794,7 @@ jobs:
 
           docker run --rm `
             -e PYTHONPATH=C:\workspace `
+            -e PYTHONIOENCODING=utf-8 `
             -v "$($PWD.Path):C:\workspace" `
             -w "C:\workspace\book\quarto\${{ matrix.output_dir }}" `
             ${{ env.CONTAINER_IMAGE }} `