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>

heiniremote.flatpakrepo

@@ -0,0 +1,9 @@
+[Flatpak Repo]
+Title=HeiniRemote Agent
+# The CI job regenerates this file on the 'pages' branch with the correct URL,
+# so the canonical copy to use is:
+#   https://git.pc-heini.de/pc-heini/heiniremote-flatpak/raw/branch/pages/heiniremote.flatpakrepo
+Url=https://git.pc-heini.de/pc-heini/heiniremote-flatpak/raw/branch/pages
+Homepage=https://pc-heini.de/
+Comment=HeiniRemote remote maintenance agent as a Flatpak
+Description=Install the HeiniRemote agent on any Linux distribution via Flatpak.