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

DECISIONS.md

@@ -1,5 +1,12 @@
 # Decisions
 
+## 2026-06-02 – Inline power confirmation before reboot/shutdown (v0.10.0)
+
+- **Who**: ClaudeCode, Dom
+- **Why**: Reboot/Shutdown buttons triggered the action immediately on click — one misclick rebooted the machine from the greeter. moonlock already guards power actions with an inline confirm; moongreet should match.
+- **Tradeoffs**: Ported moonlock's `show_power_confirm`/`dismiss_power_confirm` verbatim instead of inventing a new widget — keeps the two codebases symmetric (i18n, CSS classes, focus-on-Cancel behaviour all identical). Dropped the `button` parameter from `execute_power_action`: the old per-button `set_sensitive(false)` double-click guard is now redundant because the confirm box itself blocks accidental re-trigger, and after "Yes" there is no button left to re-enable.
+- **How**: Inline confirm box appended to the central `login_box` (mirrors moonlock placement). Reboot/Shutdown handlers call `show_power_confirm`; "Yes" dismisses and runs the action, "Cancel" (focused by default) just dismisses. New i18n strings (`reboot_confirm`, `shutdown_confirm`, `confirm_yes`, `confirm_no`) and `.confirm-*` CSS classes ported from moonlock; `.confirm-no` background adapted to moongreet's `alpha(@theme_fg_color, …)` idiom.
+
 ## 2026-06-02 – Cursor theme via GtkSettings, salvaged from unpushed work (v0.9.0)
 
 - **Who**: ClaudeCode, Dom