Document and package the new libjpeg-turbo dependency

Tight's JPEG sub-mode (previous commit) links libjpeg-turbo directly.
Adds it to docs/BUILDING.md for all three platforms (apt
libjpeg-turbo8-dev, brew jpeg-turbo, vcpkg libjpeg-turbo:x64-windows)
and to the .deb package's Depends: line (libjpeg-turbo8). No changes
needed for Windows (the Inno Setup script already wildcards *.dll) or
macOS (macdeployqt bundles non-system dylibs automatically). Left the
Flatpak manifests unchanged on the assumption that the KDE runtime
already bundles libjpeg-turbo as a standard Qt JPEG-plugin dependency
-- flagged in PROGRESS.md as worth confirming next time a Flatpak
build actually runs, since that wasn't independently verified here.

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

packaging/linux/build-deb.sh

@@ -39,7 +39,7 @@ Section: net
 Priority: optional
 Architecture: ${ARCH}
 Maintainer: OrbitHub Maintainers <maintainers@orbithub.local>
-Depends: libc6, libstdc++6, libqt6core6, libqt6gui6, libqt6widgets6, libqt6sql6, libssl3, zlib1g, openssh-client
+Depends: libc6, libstdc++6, libqt6core6, libqt6gui6, libqt6widgets6, libqt6sql6, libssl3, zlib1g, libjpeg-turbo8, openssh-client
 Description: OrbitHub remote session manager
  OrbitHub is a native desktop application for managing connection profiles
  and opening SSH and RDP sessions in a tabbed interface.