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.

resources/style.css

@@ -0,0 +1,105 @@
+/* ABOUTME: GTK4 CSS stylesheet for the Moonset power menu. */
+/* ABOUTME: Uses GTK theme colors for consistency with the active desktop theme. */
+
+/* Main panel window background */
+window.panel {
+    background-color: @theme_bg_color;
+    background-size: cover;
+    background-position: center;
+}
+
+/* Wallpaper-only window for secondary monitors */
+window.wallpaper {
+    background-color: @theme_bg_color;
+}
+
+/* Round avatar image */
+.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;
+}
+
+/* Action button — square card */
+.action-button {
+    min-width: 120px;
+    min-height: 120px;
+    padding: 16px;
+    border-radius: 50%;
+    background-color: alpha(@theme_base_color, 0.55);
+    color: @theme_fg_color;
+    border: none;
+}
+
+.action-button:hover {
+    background-color: alpha(@theme_base_color, 0.7);
+}
+
+/* Action icon inside button — request 48px from theme, scale up via CSS */
+.action-icon {
+    color: @theme_fg_color;
+    -gtk-icon-size: 64px;
+}
+
+/* Action label below icon */
+.action-label {
+    font-size: 14px;
+    color: @theme_unfocused_fg_color;
+}
+
+/* Confirmation box below action buttons */
+.confirm-box {
+    padding: 16px 24px;
+    background-color: transparent;
+}
+
+/* Confirmation prompt text */
+.confirm-label {
+    font-size: 16px;
+    color: @theme_fg_color;
+    margin-bottom: 4px;
+}
+
+/* Confirm "Yes" button */
+.confirm-yes {
+    padding: 8px 24px;
+    border-radius: 8px;
+    background-color: @error_color;
+    color: @theme_bg_color;
+    border: none;
+    font-weight: bold;
+}
+
+.confirm-yes:hover {
+    background-color: lighter(@error_color);
+}
+
+/* Confirm "No/Cancel" button */
+.confirm-no {
+    padding: 8px 24px;
+    border-radius: 8px;
+    background-color: @theme_unfocused_bg_color;
+    color: @theme_fg_color;
+    border: none;
+}
+
+.confirm-no:hover {
+    background-color: @theme_selected_bg_color;
+}
+
+/* Error message label */
+.error-label {
+    color: @error_color;
+    font-size: 14px;
+}