Rewrite moongreet from Python to Rust (v0.3.0)

Complete rewrite of the greetd greeter from Python/PyGObject to Rust/gtk4-rs
for consistency with moonset, single binary without Python runtime, and
improved security through Rust memory safety.

Modules: main, greeter, ipc, config, i18n, users, sessions, power
86 unit tests covering all modules including login_worker IPC flow.
Security hardening: shell-word splitting for exec_cmd, absolute path
validation for session binaries, session-name sanitization, absolute
loginctl path, atomic IPC writes.

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",
+        "moongreet.gresource",
+    );
+}