refactor: power-confirm via PowerAction table (v0.10.1)

Replace the two hand-wired reboot/shutdown handlers and the loose-param
show_power_confirm with a PowerAction table + create_power_button factory,
mirroring moonset's ActionDef pattern. Couples icon/prompt/error/action so
a mismatched prompt/action pair is unrepresentable.

Restore the in-flight re-trigger guard via power_box.set_sensitive(false)
(re-enabled on failure), superseding the v0.10.0 "no guard" tradeoff.

Cargo.lock

@@ -575,7 +575,7 @@ dependencies = [
 
 [[package]]
 name = "moongreet"
-version = "0.10.0"
+version = "0.10.1"
 dependencies = [
  "gdk-pixbuf",
  "gdk4",