Fix Windows backup: Handle reserved device names (nul, con, prn, etc.)

Windows has reserved device names that cannot be used as file names:
- CON, PRN, AUX, NUL
- COM1-9, LPT1-9

Changed backup function from Copy-Item to Robocopy which properly
handles these reserved names. This fixes the error when backing up
.claude directories that contain files with these names.

Robocopy exit codes:
- 0-7 = Success
- 8+ = Errors

Co-Authored-By: Claude <noreply@anthropic.com>

install-windows.ps1

@@ -150,8 +150,14 @@ function Backup-ExistingConfig {
     $backupDir = Join-Path $env:USERPROFILE ".claude-backup-$(Get-Date -Format 'yyyyMMdd_HHmmss')"
 
     if (Test-Path $claudeDir) {
-        Copy-Item -Path $claudeDir -Destination $backupDir -Recurse -Force
+        # Use Robocopy to handle reserved device names (nul, con, prn, etc.)
+        $robocopyResult = robocopy $claudeDir $backupDir /E /R:0 /W:0 /NFL /NDL /NJH /NJS 2>&1
+
+        if ($LASTEXITCODE -lt 8) {
             Write-ColorOutput "Backup created at: $backupDir" -Type Success
+        } else {
+            Write-ColorOutput "Backup had some issues (continued anyway)." -Type Warning
+        }
     } else {
         Write-ColorOutput "No existing configuration to backup." -Type Info
     }