fix(windows): address code review issues

- Fix openInExplorer to actually execute explorer.exe (was a no-op)
- Load System.Windows.Forms assembly before using Clipboard
- Use Remove-SafeItem in purge for consistent safety checks
- Fix Dropbox typo (was DroplboxCache)

windows/lib/clean/user.ps1

@@ -289,6 +289,9 @@ function Clear-ClipboardHistory {
     }
     
     try {
+        # Load Windows Forms assembly for clipboard access
+        Add-Type -AssemblyName System.Windows.Forms -ErrorAction SilentlyContinue
+        
         # Clear current clipboard
         [System.Windows.Forms.Clipboard]::Clear()