launcher: side-by-side versions.json fallback + empty-state UX

The version dropdown sat silently empty when the GitHub manifest
fetch 404'd (private repo, network down, etc.) and there was no
DORKNET_LOCAL_MANIFEST to fall back to. Two fixes:

- csproj copies the repo-root versions.json next to dorknet.exe on
  build, and LocalOverrides.GetLocalManifestPath checks that path
  as a fallback after the env var. Dev checkouts against a private
  repo now Just Work — no env vars needed.
- When the manifest can't be loaded, RefreshManifestAsync now
  inserts a disabled placeholder "(no versions - check internet or
  set DORKNET_LOCAL_MANIFEST)" in the dropdown so the user sees the
  problem at a glance instead of a silent blank.
- LocalOverrides.DescribeManifestSource shows in Settings whether
  the active manifest came from the env var, the side-by-side
  fallback, or the live GitHub fetch.

launcher/DorkNet.Launcher.csproj

@@ -27,4 +27,17 @@
     <PackageReference Include="QRCoder" Version="1.6.0" />
   </ItemGroup>
 
+  <!-- Dev convenience: copy the repo-root versions.json next to dorknet.exe
+       on build, so a dev checkout works against the private repo without
+       needing DORKNET_LOCAL_MANIFEST set. LocalOverrides picks this up as
+       a side-by-side fallback. The file is harmless to ship in release
+       builds too — VersionsManifest still prefers the live GitHub fetch
+       when reachable. -->
+  <ItemGroup>
+    <None Include="..\versions.json" Condition="Exists('..\versions.json')">
+      <Link>versions.json</Link>
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
+  </ItemGroup>
+
 </Project>