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.

flathub/.gitignore

@@ -0,0 +1,14 @@
+# Generated files - do not commit to main repo
+# These are generated by script/prepare-flathub
+vendor/
+vendor.tar.gz
+su.reya.signed.yml
+su.reya.signed.metainfo.xml
+release-info.xml
+cargo-config.toml
+build/
+repo/
+
+# Keep the README and this .gitignore
+!README.md
+!.gitignore