launcher: DORKNET_LOCAL_* env-var overrides for dev testing

Set DORKNET_LOCAL_MANIFEST and/or DORKNET_LOCAL_RELEASES to point
the launcher at on-disk artifacts instead of the public repo:

- DORKNET_LOCAL_MANIFEST: full path to a versions.json file.
  VersionsManifest.FetchAsync reads it from disk and skips the
  GitHub round-trip.
- DORKNET_LOCAL_RELEASES: path to a directory laid out as
  {root}/{version_key}/server/ + .../patcher/. EnsureServerAsync /
  EnsurePatcherAsync return those paths in-place — no download,
  no copy, no marker files.

Both are silent no-ops when unset / missing on disk, so it's safe
to leave them set permanently in a dev profile. When at least one
is active, the Settings view surfaces a "Dev overrides active"
panel listing the paths so it's obvious the launcher isn't using
the public manifest.

Lets contributors iterate on per-version branch changes against
the launcher UI without needing to tag a public Release.

launcher/MainWindow.xaml.cs

@@ -166,6 +166,17 @@ public partial class MainWindow : Window
             $"Patchers: {AppPaths.PatchersRoot}\n" +
             $"Logs:    {AppPaths.LogsRoot}";
 
+        var overrides = LocalOverrides.DescribeActive();
+        if (!string.IsNullOrEmpty(overrides))
+        {
+            LocalOverridesPanel.Visibility = Visibility.Visible;
+            LocalOverridesText.Text = overrides;
+        }
+        else
+        {
+            LocalOverridesPanel.Visibility = Visibility.Collapsed;
+        }
+
         RefreshJoinReadiness();
         ShowView(PickRoutingTarget());
     }