fix: power buttons via systemctl, single greeter window (v0.8.7)

Reboot/shutdown buttons always failed: power.rs called `loginctl
reboot|poweroff`, but loginctl has no such verbs (systemd 260) — those
belong to systemctl. moonlock/moonset already used systemctl; moongreet
was the outlier. Switch to `systemctl --no-ask-password reboot|poweroff`.

The multi-monitor greeter gave Exclusive keyboard only to the first
monitor's window, so a user focused on any other output could not type
the password. Drop the per-monitor loop + hotplug; create one window on
the focused output (no set_monitor) with Exclusive keyboard.

Polkit rule kept as a harmless safety net (it was never the blocker;
CanReboot returns yes). The missing journal errors were not a logging
bug — they were lost to a hard power-cut before journald synced.

DECISIONS.md

@@ -1,5 +1,12 @@
 # Decisions
 
+## 2026-06-02 – Power buttons fixed (loginctl→systemctl) + single greeter window (v0.8.7)
+
+- **Who**: ClaudeCode, Dom
+- **Why**: At the greeter the reboot and shutdown buttons always failed with "Neustart/Herunterfahren fehlgeschlagen". Root cause: `power.rs` invoked `/usr/bin/loginctl reboot|poweroff`, but `loginctl` has no such verbs (systemd 260: `Unknown command verb 'reboot'`, exit 1) — power-management verbs belong to `systemctl`. moonlock and moonset already used `systemctl`; moongreet was the outlier (moonset carried the same bug until Mar 29). The polkit rule shipped in v0.8.3 treated the wrong layer — `CanReboot` returns `yes`, polkit was never the blocker. Separately, the multi-monitor greeter (v0.8.0/v0.8.2) gave `KeyboardMode::Exclusive` to only the first enumerated monitor's window, so on a multi-output setup the user could not type the password when focused on any other output.
+- **Tradeoffs**: Dropping the per-monitor + hotplug windows leaves secondary monitors blank during login; irrelevant for a login screen (input happens on one output). Exclusive keyboard binds input to the single greeter surface regardless of pointer position — the mouse may wander to a blank output but typing always reaches the greeter (chosen over compositor-level pointer confinement). The polkit rule is kept as a harmless safety net for the agent-less greeter session; its misleading "session is inactive" comment was corrected.
+- **How**: (1) `power::reboot`/`shutdown` call `/usr/bin/systemctl --no-ask-password reboot|poweroff` (matches moonlock; `--no-ask-password` fails fast instead of hanging on a missing askpass agent). (2) `main.rs` `activate()` creates one greeter window with no `set_monitor` (compositor places it on the focused output, like moonset) and `KeyboardMode::Exclusive`; the monitor loop, `connect_items_changed` hotplug handler, and the now-unused `glib::clone`/`std::rc::Rc` imports are removed. (3) The missing journal entries were investigated and are **not** a logging bug — the greeter user delivers all priorities to journald (verified live); the two button errors were lost because boot -2 was hard-cut before journald's 5-minute sync.
+
 ## 2026-04-24 – Audit LOW fixes: stdout null, utf-8 path, debug value, hidden sessions (v0.8.6)
 
 - **Who**: ClaudeCode, Dom