Add Flatpak packaging for Patchwork 5.4.0

Bundle Poncho Wonky (Patchwork 5.4.0) for x86_64 and aarch64 on the
24.08 freedesktop runtime with the Electron BaseApp. Launcher forces
Wayland when a socket is present so the app starts from terminals
outside the desktop session. Pinned versions and checksums live in
versions.sh and are validated by check-versions.sh.

versions.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+# Single source of truth for every pinned version in this packaging.
+# build.sh sources this file; check-versions.sh verifies the manifest and
+# metainfo still match. Bump versions here first, then run check-versions.sh.
+
+# Flatpak runtime/SDK/Electron BaseApp branch. These three always move
+# together. The app bundles its own Electron binary from the tar.gz, so this
+# only needs to satisfy the host glibc/GTK stack Electron requires -- bump to
+# the next branch (e.g. 25.08) only when a newer Electron demands it.
+export RUNTIME_VERSION="24.08"
+
+# Upstream Patchwork (Poncho Wonky) release.
+export APP_VERSION="5.4.0"
+export APP_SHA256_X86_64="15b147f35381b4e3bd1a7b5b3b9bfe924c6427c65e5ff72c25209ddf93141e7a"
+export APP_SHA256_AARCH64="c78bb91f12c55bc72d68ff8a4e90a162eefd55010d2686443a23de808901f70b"