Windows: embed app icon into the executable file

The app's icon only appeared at runtime (window/taskbar), set programmatically
via QApplication::setWindowIcon(). Explorer showed the generic default icon
for orbithub.exe itself, since that requires an icon baked into the PE file
as a Windows resource at build time -- a completely separate mechanism.

Added packaging/windows/orbithub.ico, rendered directly from the same
createOrbitHubAppIcon() logic used at runtime (via a one-off export tool,
not committed) so the file icon matches what the app actually looks like.
A minimal .rc resource file embeds it, compiled in only for WIN32 builds.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packaging/windows/orbithub.rc

@@ -0,0 +1 @@
+1 ICON "orbithub.ico"