feat: Flatpak packaging for the HeiniRemote agent

Bundles the agent as de.pc_heini.HeiniRemoteAgent on the freedesktop 24.08
runtime, mirroring the o3de-flatpak CI approach (bwrap-free build, OSTree
repo force-pushed to pages, single-file bundle attached to a Gitea release).
Compiles the agent from upstream source (newest v* tag, else main).

Sandbox notes: screen capture/input via portals need no special permission;
the PTY console uses flatpak-spawn --host for a real host shell, and a native
install's agent.toml is adopted read-only.

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

de.pc_heini.HeiniRemoteAgent.desktop

@@ -0,0 +1,9 @@
+[Desktop Entry]
+Type=Application
+Name=HeiniRemote Agent
+Comment=Remote maintenance agent — connects this machine to a HeiniRemote server
+Exec=heiniremote-wrapper.sh
+Icon=de.pc_heini.HeiniRemoteAgent
+Terminal=false
+Categories=Network;RemoteAccess;
+Keywords=remote;support;fernwartung;