Desktop installer: release channel, saved choices, robust delete, logs

- Channel radio (Release / Master (Instabil)); master installs the
  rolling 'master' release from OmniNX/OmniNX-Master. Preselected from
  release_channel in the card's manifest.ini.
- Variant, RAM (8gb) and Hekate-Pro choice are prefilled from the card
  (manifest.ini / config.ini) like the payload does, instead of
  silently switching users back to 4 GB / Standard.
- Clear read-only attributes before deleting folders.
- Remove leftover "OmniNX <Variant>" pack folders after installing.
- Create empty folders from the pack.
- Save every run's log to %LocalAppData%\OmniNX-Desktop-Installer\logs,
  plus a "Save log" button.

desktop/OmniNX.DesktopInstaller/Models/InstallModels.cs

@@ -8,6 +8,12 @@ public enum OmniNxVariant
     Oc,
 }
 
+public enum ReleaseChannel
+{
+    Release,
+    Master,
+}
+
 public enum InstallMode
 {
     Clean,
@@ -30,6 +36,7 @@ public sealed class OmniNxStatus
     public bool IsInstalled { get; init; }
     public OmniNxVariant Variant { get; init; }
     public string Version { get; init; } = string.Empty;
+    public ReleaseChannel Channel { get; init; } = ReleaseChannel.Release;
 }
 
 public sealed class ReleaseInfo