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/style.css

@@ -0,0 +1,92 @@
+/* ABOUTME: GTK4 CSS stylesheet for the Moongreet greeter. */
+/* ABOUTME: Defines styling for the login screen layout. */
+
+/* Main window background */
+window.greeter {
+    background-color: #1a1a2e;
+    background-size: cover;
+    background-position: center;
+}
+
+/* Wallpaper-only window for secondary monitors */
+window.wallpaper {
+    background-color: #1a1a2e;
+}
+
+/* Central login area */
+.login-box {
+    padding: 40px;
+    border-radius: 12px;
+    background-color: transparent;
+}
+
+/* Round avatar image — size is set via set_size_request() in code */
+.avatar {
+    border-radius: 50%;
+    min-width: 128px;
+    min-height: 128px;
+    background-color: @theme_selected_bg_color;
+    border: 3px solid alpha(white, 0.3);
+}
+
+/* Username label */
+.username-label {
+    font-size: 24px;
+    font-weight: bold;
+    color: white;
+    margin-top: 12px;
+    margin-bottom: 40px;
+}
+
+/* Session dropdown */
+.session-dropdown {
+    min-width: 280px;
+}
+
+/* Password entry field */
+.password-entry {
+    min-width: 280px;
+}
+
+/* Error message label */
+.error-label {
+    color: #ff6b6b;
+    font-size: 14px;
+}
+
+/* User list on the bottom left */
+.user-list {
+    background-color: transparent;
+    padding: 8px;
+}
+
+.user-list-item {
+    padding: 8px 16px;
+    border-radius: 8px;
+    color: white;
+    font-size: 14px;
+}
+
+.user-list-item:hover {
+    background-color: alpha(white, 0.15);
+}
+
+.user-list-item:selected {
+    background-color: alpha(white, 0.2);
+}
+
+/* Power buttons on the bottom right */
+.power-button {
+    min-width: 48px;
+    min-height: 48px;
+    padding: 0px;
+    border-radius: 24px;
+    background-color: alpha(white, 0.1);
+    color: white;
+    border: none;
+    margin: 4px;
+}
+
+.power-button:hover {
+    background-color: alpha(white, 0.25);
+}