feat: rewrite moonset in Rust (gtk4-rs + gtk4-layer-shell)

Feature-parity with Python v0.2.0. Same CSS, same UI, same actions.
Single 3.1 MB binary with embedded resources (CSS, wallpaper, avatar).

Modules: power.rs, i18n.rs, config.rs, users.rs, panel.rs, main.rs
45 unit tests passing. Python sources retained as reference.

Cargo.toml

@@ -0,0 +1,25 @@
+[package]
+name = "moonset"
+version = "0.1.0"
+edition = "2024"
+description = "Wayland session power menu with GTK4 and Layer Shell"
+license = "MIT"
+
+[dependencies]
+gtk4 = { version = "0.11", features = ["v4_10"] }
+gtk4-layer-shell = "0.8"
+glib = "0.22"
+gdk4 = "0.11"
+gdk-pixbuf = "0.22"
+toml = "0.8"
+dirs = "6"
+serde = { version = "1", features = ["derive"] }
+nix = { version = "0.29", features = ["user"] }
+log = "0.4"
+env_logger = "0.11"
+
+[dev-dependencies]
+tempfile = "3"
+
+[build-dependencies]
+glib-build-tools = "0.22"