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.toml

@@ -1,6 +1,6 @@
 [package]
 name = "moongreet"
-version = "0.10.0"
+version = "0.10.1"
 edition = "2024"
 description = "A greetd greeter for Wayland with GTK4 and Layer Shell"
 license = "MIT"