feat: confirm dialog before power actions (v0.10.0)

Reboot/shutdown buttons triggered immediately on click. Now show an
inline confirmation prompt, mirroring moonlock's show_power_confirm.

- i18n: reboot_confirm, shutdown_confirm, confirm_yes, confirm_no (DE/EN)
- greeter: confirm_area in login_box, handlers route through
  show_power_confirm/dismiss_power_confirm; execute_power_action drops
  the now-redundant button-disable guard
- style: .confirm-label/-yes/-no classes

resources/style.css

@@ -82,6 +82,38 @@ window.wallpaper {
     background-color: alpha(@theme_fg_color, 0.2);
 }
 
+/* Power confirmation prompt */
+.confirm-label {
+    font-size: 16px;
+    color: @theme_fg_color;
+    margin-bottom: 4px;
+}
+
+.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 {
+    padding: 8px 24px;
+    border-radius: 8px;
+    background-color: alpha(@theme_fg_color, 0.15);
+    color: @theme_fg_color;
+    border: none;
+}
+
+.confirm-no:hover {
+    background-color: alpha(@theme_fg_color, 0.25);
+}
+
 /* Power buttons on the bottom right */
 .power-button {
     min-width: 48px;