chore: add release infrastructure and packaging support

Add GitHub Actions workflows for CI and release builds, packaging
scripts for Windows, macOS, Linux, and Flatpak/Snap distribution,
application icons, desktop files, and release metadata resources.

script/try-snap

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+
+# This script is intended to be run after `script/bundle-snap`.
+#
+# It expects a version to be passed as the first argument, and expects the
+# built `.snap` for that version to be in the current directory. bundle-snap
+# names it `signed_<version>_amd64.snap` (Snap's canonical amd64/arm64 naming).
+#
+# This will uninstall the current `signed` snap, replacing it with a snap
+# that directly uses the `snap/unpacked` directory.
+
+set -euxo pipefail
+
+if [ "$#" -ne 1 ]; then
+  echo "Usage: $0 <release_version>"
+  exit 1
+fi
+
+# Rerun as root
+[ "$UID" -eq 0 ] || exec sudo bash -e "$0" "$@"
+
+snap remove signed || true
+mkdir -p snap
+rm -rf snap/unpacked
+unsquashfs -dest snap/unpacked "signed_$1_amd64.snap"
+snap try --classic snap/unpacked