v1.15 — PowerShell edition + repo restructure

- New powershell/InstaSoftOfficeTool.ps1: single-file WPF GUI version
  - Same Fluent Design UI, no compilation needed
  - Runs on any Windows 7+ with PowerShell 5.1 (built-in)
  - Chrome won't flag .ps1 files as "rarely downloaded"
  - Auto-elevates to admin
- Moved C# source to src/ subfolder
- Updated .gitignore for nested bin/obj folders

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

Pages/WelcomePage.xaml.cs

@@ -1,31 +0,0 @@
-using System.Windows;
-using System.Windows.Controls;
-
-namespace InstaSoftOfficeTool.Pages
-{
-    public partial class WelcomePage : Page
-    {
-        private readonly MainWindow _main;
-
-        public WelcomePage(MainWindow main)
-        {
-            InitializeComponent();
-            _main = main;
-        }
-
-        private void InstallClick(object sender, RoutedEventArgs e)
-        {
-            _main.StartInstallFlow();
-        }
-
-        private void RemoveClick(object sender, RoutedEventArgs e)
-        {
-            _main.StartRemoveFlow();
-        }
-
-        private void LicenseClick(object sender, RoutedEventArgs e)
-        {
-            _main.StartLicenseFlow();
-        }
-    }
-}