feat: add battery conservation mode with Waybar toggle

Laptops with charge_control_end_threshold support get a click-to-toggle
on the battery module (80% ↔ 100%). A ♥ icon appears when conservation
is active, hidden when inactive. State persists across reboots via
systemd oneshot service. udev rule grants wheel group write access
so no sudo is needed for toggling.

defaults/etc/systemd/system/moonarch-batsaver.service

@@ -0,0 +1,15 @@
+# ABOUTME: Restores battery charge threshold from saved state on boot.
+# ABOUTME: Only runs on laptops with threshold support and a saved state file.
+
+[Unit]
+Description=Restore battery conservation mode threshold
+After=sysinit.target
+ConditionPathExists=/sys/class/power_supply/BAT0/charge_control_end_threshold
+ConditionPathExists=/var/lib/moonarch/batsaver-threshold
+
+[Service]
+Type=oneshot
+ExecStart=/bin/sh -c 'cat /var/lib/moonarch/batsaver-threshold > /sys/class/power_supply/BAT0/charge_control_end_threshold'
+
+[Install]
+WantedBy=multi-user.target