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.

resources/resources.gresource.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<gresources>
+  <gresource prefix="/dev/moonarch/moongreet">
+    <file>style.css</file>
+    <file>wallpaper.jpg</file>
+    <file>default-avatar.svg</file>
+  </gresource>
+</gresources>