packaging: add deb/flatpak build flow and desktop metadata

docs/BUILDING.md

@@ -84,3 +84,24 @@ If you already have `Qt 6` from the Qt installer and do not want vcpkg Qt, you c
 - Build output executable:
   - Linux/macOS: `build/orbithub`
   - Windows: `build\\orbithub.exe`
+
+## Linux Packaging
+
+Build a Debian package (`.deb`) from the current Linux build:
+
+```bash
+./packaging/linux/build-deb.sh
+```
+
+Output path:
+- `dist/orbithub_<version>_<arch>.deb`
+
+Build a Flatpak bundle:
+
+```bash
+sudo apt-get install -y flatpak flatpak-builder
+./packaging/flatpak/build-flatpak.sh
+```
+
+Output path:
+- `dist/flatpak/io.orbithub.OrbitHub.flatpak`