launcher: QR code + share buttons on host ready panel

The single Copy button left it on the user to format a message and
get the code into Discord/email. Added a layout that makes sharing
obvious:

- QR encodes the join code text so a friend with a phone can scan
  the host's screen and forward the code without typing it
- "Copy as message" formats a one-line "Join {server} on DorkNet!
  Download at github.com/... and paste this code:" pitch
- "Email to friend" opens the default mail client with subject +
  body prefilled, code in the body
- QR built via QRCoder -> WriteableBitmap directly (no
  System.Drawing dependency); palette matches the launcher

launcher/DorkNet.Launcher.csproj

@@ -20,4 +20,11 @@
        The Backend/* classes (release downloader, server process, etc.) are
        UI-framework-agnostic and stay unchanged from the PhotinoNET draft. -->
 
+  <ItemGroup>
+    <!-- QR rendering for the share panel — generates the code as raw
+         module bytes, MainWindow draws it onto a WPF Image. No System.Drawing
+         dependency that way, so it stays portable. -->
+    <PackageReference Include="QRCoder" Version="1.6.0" />
+  </ItemGroup>
+
 </Project>