packaging: rename app ID to org.darksingularity.OrbitHub

The reverse-DNS app ID should reflect a domain we actually control.
Renames the desktop file, AppStream metainfo, icon, Flatpak manifest,
and macOS bundle identifier from io.orbithub.OrbitHub to
org.darksingularity.OrbitHub. Also fixes a stale homepage URL in the
metainfo file pointing at an old git host.

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

packaging/linux/org.darksingularity.OrbitHub.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Type=Application
+Version=1.0
+Name=OrbitHub
+GenericName=Remote Session Manager
+Comment=Manage SSH and RDP sessions in one native app
+Exec=orbithub
+Icon=org.darksingularity.OrbitHub
+Terminal=false
+Categories=Network;RemoteAccess;Utility;
+StartupNotify=true