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>

docs/BUILDING.md

@@ -105,4 +105,4 @@ sudo apt-get install -y flatpak flatpak-builder
 ```
 
 Output path:
-- `dist/flatpak/io.orbithub.OrbitHub.flatpak`
+- `dist/flatpak/org.darksingularity.OrbitHub.flatpak`