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.

build.rs

@@ -0,0 +1,10 @@
+// ABOUTME: Build script for compiling GResource bundle.
+// ABOUTME: Bundles style.css, wallpaper.jpg, and default-avatar.svg into the binary.
+
+fn main() {
+    glib_build_tools::compile_resources(
+        &["resources"],
+        "resources/resources.gresource.xml",
+        "moonset.gresource",
+    );
+}