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/get-crate-version

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+
+set -eu
+
+if [[ $# -ne 1 ]]; then
+  echo "Usage: $0 <crate_name>" >&2
+  exit 1
+fi
+
+CRATE_NAME=$1
+
+cargo metadata \
+    --no-deps \
+    --format-version=1 \
+    | jq \
+        --raw-output \
+        ".packages[] | select(.name == \"${CRATE_NAME}\") | .version"