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.

.gitignore

@@ -1 +1,20 @@
-/target
+# Generated by Cargo
+# will have compiled files and executables
+debug/
+target/
+dist/
+
+# Snap and Flatpak local build output
+/snap
+/su.reya.signed.json
+/linux-artifacts
+
+# Vendored dependencies + cargo config generated by script/prepare-flathub
+.cargo/
+vendor/
+
+# MSVC Windows builds of rustc generate these, which store debugging information
+*.pdb
+
+# macOS
+.DS_Store