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/Backend/VersionsManifest.cs

@@ -30,6 +30,23 @@ public sealed class VersionsManifest
 
     public async Task<VersionsManifest?> FetchAsync(CancellationToken ct = default)
     {
+        // Dev-time override: read manifest from a local file instead of
+        // GitHub. Used for testing branch changes before they're public.
+        var localPath = LocalOverrides.GetLocalManifestPath();
+        if (localPath is not null)
+        {
+            try
+            {
+                var json = await File.ReadAllTextAsync(localPath, ct);
+                return JsonSerializer.Deserialize<VersionsManifest>(json);
+            }
+            catch
+            {
+                // Bad local file — fall through to the network so the
+                // user gets a real error instead of silent emptiness.
+            }
+        }
+
         try
         {
             var m = await Http.GetFromJsonAsync<VersionsManifest>(ManifestUrl, ct);