Switch to .NET 8 for cross-platform build support

- Target net8.0-windows instead of net4.8
- EnableWindowsTargeting for Linux build
- Replace Marshal.GetActiveObject with P/Invoke (not in .NET 8)
- Use NuGet package for Outlook Interop instead of local DLL ref
- Update Inno Setup script for .NET 8 runtime check
- Builds successfully on Linux, runs on Windows

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

src/StarfaceOutlookSync/Services/OutlookContactsService.cs

@@ -11,6 +11,17 @@ namespace StarfaceOutlookSync.Services
         private Outlook.Application _outlookApp;
         private bool _weStartedOutlook;
 
+        // Marshal.GetActiveObject existiert nicht in .NET 8, daher P/Invoke
+        [DllImport("oleaut32.dll", PreserveSig = false)]
+        private static extern void GetActiveObject([MarshalAs(UnmanagedType.LPStruct)] Guid rclsid, IntPtr pvReserved, [MarshalAs(UnmanagedType.IUnknown)] out object ppunk);
+
+        private static object GetActiveComObject(string progId)
+        {
+            var clsid = Type.GetTypeFromProgID(progId, true).GUID;
+            GetActiveObject(clsid, IntPtr.Zero, out var obj);
+            return obj;
+        }
+
         private Outlook.Application GetOutlookApp()
         {
             if (_outlookApp != null) return _outlookApp;
@@ -18,7 +29,7 @@ namespace StarfaceOutlookSync.Services
             try
             {
                 // Versuche laufende Outlook-Instanz zu finden
-                _outlookApp = (Outlook.Application)Marshal.GetActiveObject("Outlook.Application");
+                _outlookApp = (Outlook.Application)GetActiveComObject("Outlook.Application");
                 _weStartedOutlook = false;
             }
             catch