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/UI/MainForm.cs

@@ -228,17 +228,17 @@ namespace StarfaceOutlookSync.UI
             }
         }
 
-        private async Task SyncSelectedProfile()
+        private Task SyncSelectedProfile()
         {
             if (_profileList.SelectedItems.Count == 0)
             {
                 MessageBox.Show("Bitte ein Profil auswaehlen.", "Sync",
                     MessageBoxButtons.OK, MessageBoxIcon.Information);
-                return;
+                return Task.CompletedTask;
             }
 
             var profile = _profileList.SelectedItems[0].Tag as SyncProfile;
-            if (profile == null) return;
+            if (profile == null) return Task.CompletedTask;
 
             using (var syncForm = new SyncProgressForm(profile))
             {
@@ -246,6 +246,7 @@ namespace StarfaceOutlookSync.UI
             }
 
             RefreshProfileList();
+            return Task.CompletedTask;
         }
 
         private async Task RunSync(SyncProfile profile)