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/bin/moonarch-batsaver-toggle

@@ -0,0 +1,28 @@
+#!/usr/bin/bash
+# ABOUTME: Toggles battery conservation mode between 80% and 100% charge limit.
+# ABOUTME: Writes to sysfs (immediate) and state file (persistence across reboots).
+
+THRESHOLD_FILE="/sys/class/power_supply/BAT0/charge_control_end_threshold"
+STATE_DIR="/var/lib/moonarch"
+STATE_FILE="${STATE_DIR}/batsaver-threshold"
+CONSERVATION_LIMIT=80
+
+[[ -f "$THRESHOLD_FILE" ]] || exit 1
+
+CURRENT=$(cat "$THRESHOLD_FILE")
+
+if [[ "$CURRENT" -le "$CONSERVATION_LIMIT" ]]; then
+    NEW=100
+else
+    NEW="$CONSERVATION_LIMIT"
+fi
+
+# Apply immediately
+echo "$NEW" > "$THRESHOLD_FILE" || exit 1
+
+# Persist for next boot
+mkdir -p "$STATE_DIR"
+echo "$NEW" > "$STATE_FILE"
+
+# Signal Waybar to refresh the batsaver module (SIGRTMIN+9)
+pkill -RTMIN+9 waybar