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.metainfo.xml

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<component type="desktop-application">
+  <id>de.pc_heini.HeiniRemoteAgent</id>
+  <metadata_license>CC0-1.0</metadata_license>
+  <project_license>proprietary</project_license>
+  <name>HeiniRemote Agent</name>
+  <summary>Remote maintenance agent for HeiniRemote</summary>
+  <description>
+    <p>
+      Customer-side agent for HeiniRemote: Wayland screen capture and input
+      injection via xdg-desktop-portal, plus a PTY rescue console, streamed
+      over WebRTC to the HeiniRemote management console. Requires a HeiniRemote
+      server URL and device token (agent.toml).
+    </p>
+  </description>
+  <launchable type="desktop-id">de.pc_heini.HeiniRemoteAgent.desktop</launchable>
+  <url type="homepage">https://pc-heini.de/</url>
+  <developer_name>PC-Heini</developer_name>
+  <releases>
+    <!-- Version/date are stamped by CI (build-flatpak.yml) before export. -->
+    <release version="0.0.0" date="1970-01-01"/>
+  </releases>
+  <content_rating type="oars-1.1"/>
+</component>