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/LocalOverrides.cs

@@ -0,0 +1,80 @@
+using System.IO;
+
+namespace DorkNet.Launcher.Backend;
+
+/// <summary>Local dev-time overrides that bypass the GitHub-backed
+/// manifest + releases fetches. Set via environment variables before
+/// launching <c>dorknet.exe</c>:
+///
+/// <list type="bullet">
+///   <item><c>DORKNET_LOCAL_MANIFEST</c> = full path to a
+///   <c>versions.json</c> file. When set + exists,
+///   <see cref="VersionsManifest.FetchAsync"/> reads it from disk and
+///   skips the GitHub round-trip.</item>
+///   <item><c>DORKNET_LOCAL_RELEASES</c> = path to a directory whose
+///   children are <c>{version_key}/server/</c> and
+///   <c>{version_key}/patcher/</c>. When the launcher would normally
+///   download a server / patcher zip, it points at these in-place
+///   instead — no copying, no markers, just read the dir.</item>
+/// </list>
+///
+/// <para>Both overrides are no-ops when the env var is unset or the
+/// path doesn't exist on disk, so they're safe to leave set
+/// permanently in a dev profile.</para></summary>
+public static class LocalOverrides
+{
+    public const string ManifestEnvVar = "DORKNET_LOCAL_MANIFEST";
+    public const string ReleasesEnvVar = "DORKNET_LOCAL_RELEASES";
+
+    /// <summary>Returns the configured manifest path if the env var is
+    /// set AND the file exists; null otherwise. Caller decides whether
+    /// to fall back to the network.</summary>
+    public static string? GetLocalManifestPath()
+    {
+        var path = Environment.GetEnvironmentVariable(ManifestEnvVar);
+        if (string.IsNullOrEmpty(path)) return null;
+        return File.Exists(path) ? path : null;
+    }
+
+    /// <summary>Returns the configured releases-root if the env var is
+    /// set AND the directory exists; null otherwise.</summary>
+    public static string? GetLocalReleasesRoot()
+    {
+        var path = Environment.GetEnvironmentVariable(ReleasesEnvVar);
+        if (string.IsNullOrEmpty(path)) return null;
+        return Directory.Exists(path) ? path : null;
+    }
+
+    /// <summary>Returns the local server-dir for <paramref name="versionKey"/>
+    /// if the releases-root override is set and the subfolder exists.
+    /// Convention: <c>{root}/{version_key}/server/</c>.</summary>
+    public static string? GetLocalServerDir(string versionKey)
+    {
+        var root = GetLocalReleasesRoot();
+        if (root is null) return null;
+        var dir = Path.Combine(root, versionKey, "server");
+        return Directory.Exists(dir) ? dir : null;
+    }
+
+    /// <summary>Same as <see cref="GetLocalServerDir"/> but for the
+    /// client patcher. Convention: <c>{root}/{version_key}/patcher/</c>.</summary>
+    public static string? GetLocalPatcherDir(string versionKey)
+    {
+        var root = GetLocalReleasesRoot();
+        if (root is null) return null;
+        var dir = Path.Combine(root, versionKey, "patcher");
+        return Directory.Exists(dir) ? dir : null;
+    }
+
+    /// <summary>Single-line description of what overrides are active,
+    /// for surfacing in the Settings view. Empty string if none.</summary>
+    public static string DescribeActive()
+    {
+        var parts = new List<string>();
+        var m = GetLocalManifestPath();
+        if (m is not null) parts.Add($"manifest -> {m}");
+        var r = GetLocalReleasesRoot();
+        if (r is not null) parts.Add($"releases -> {r}");
+        return string.Join("\n", parts);
+    }
+}