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

Cargo.lock

@@ -575,7 +575,7 @@ dependencies = [
 
 [[package]]
 name = "moongreet"
-version = "0.9.0"
+version = "0.10.0"
 dependencies = [
  "gdk-pixbuf",
  "gdk4",