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.

desktop/Cargo.toml

@@ -8,6 +8,24 @@ publish.workspace = true
 name = "signed"
 path = "src/main.rs"
 
+[package.metadata.packager]
+name = "Signed"
+product-name = "Signed"
+description = "Nostr Git client for browsing repositories and collaborating"
+identifier = "su.reya.signed"
+category = "DeveloperTool"
+version = "0.1.0-alpha"
+out-dir = "../dist"
+before-packaging-command = "cargo build --release"
+resources = ["Cargo.toml", "src"]
+icons = [
+    "resources/32x32.png",
+    "resources/128x128.png",
+    "resources/128x128@2x.png",
+    "resources/icon.icns",
+    "resources/icon.ico",
+]
+
 [dependencies]
 assets = { path = "../crates/assets" }
 paths = { path = "../crates/paths" }