Fix duplicate window and cancel dialog bugs

Bug Fixes:
1. Fixed cancel button opening new window instead of exiting
   - Changed ShowInstallationScopeDialog to return bool
   - Return false on cancel, true on continue
   - Close form immediately in constructor when cancelled
   - Prevents form from showing after user cancels

2. Fixed duplicate windows when selecting system-wide installation
   - Changed from Application.Exit() to Environment.Exit(0)
   - Ensures immediate process termination after starting elevated instance
   - Prevents non-elevated instance from continuing to run
   - Only elevated instance remains open

Technical Changes:
- ShowInstallationScopeDialog now returns bool success status
- Constructor checks return value and closes form if cancelled
- Use Environment.Exit(0) for successful elevation restart
- Use Environment.Exit(1) for failed elevation
- Immediate termination prevents race conditions

Author: oop7

MainForm.cs

@@ -130,7 +130,12 @@ public MainForm()
             else
             {
                 // Show dialog for user to choose
-                ShowInstallationScopeDialog();
+                if (!ShowInstallationScopeDialog())
+                {
+                    // User cancelled - close immediately without showing form
+                    this.Load += (s, e) => this.Close();
+                    return;
+                }
             }
 
             CheckAdminPrivileges();
@@ -330,7 +335,7 @@ private void InitializeComponent()
             this.Controls.AddRange(new Control[] { headerPanel, statusLabel, speedLabel, progressBar, logTextBox, buttonPanel });
         }
 
-        private void ShowInstallationScopeDialog()
+        private bool ShowInstallationScopeDialog()
         {
             var scopeForm = new Form
             {
@@ -433,11 +438,12 @@ private void ShowInstallationScopeDialog()
                 var scopeText = installationScope == InstallationScope.User ? "user-level" : "system-wide";
                 LogMessage($"Installation scope selected: {scopeText}");
                 statusLabel.Text = $"Ready to install ({scopeText})";
+                return true;
             }
             else
             {
                 LogMessage("Installation cancelled by user");
-                Application.Exit();
+                return false;
             }
         }
 
@@ -463,15 +469,15 @@ private void CheckAdminPrivileges()
                             };
 
                             Process.Start(startInfo);
-                            Application.Exit();
+                            Environment.Exit(0); // Immediate exit to prevent duplicate windows
                         }
                         catch (Exception ex)
                         {
                             // User cancelled UAC prompt or other error
                             LogMessage($"Failed to elevate privileges: {ex.Message}");
                             MessageBox.Show("Administrator privileges are required for system-wide installation.\n\nPlease approve the UAC prompt or choose User Installation instead.",
                                 "Administrator Required", MessageBoxButtons.OK, MessageBoxIcon.Warning);
-                            Application.Exit();
+                            Environment.Exit(1);
                         }
                     }
                     else