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/bundle-macos

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+
+# Build the macOS .app/.dmg bundle with cargo-packager.
+# Requires macOS. Linux/macOS users building on Windows are unsupported here.
+
+set -euxo pipefail
+cd "$(dirname "$0")/../desktop"
+
+if ! command -v cargo-packager >/dev/null 2>&1; then
+  cargo install cargo-packager --locked
+fi
+
+cargo packager --release