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.

README.md

@@ -131,11 +131,15 @@ defaults/
     moonarch-waybar-cpugov        Waybar module: CPU governor status
     moonarch-waybar-gpustat       Waybar module: GPU utilization
     moonarch-waybar-hidpp         Waybar module: Logitech HID++ device battery
+    moonarch-waybar-batsaver      Waybar module: battery conservation mode indicator
+    moonarch-batsaver-toggle      Toggle battery charge limit (80% ↔ 100%)
 
   shell/zshrc                   Zsh config: prompt, aliases, FZF, completion
   etc/greetd/                   greetd daemon + greeter Niri config
   etc/moongreet/                moongreet configuration
   etc/systemd/user/             Systemd user services (cliphist, kanshi, walker, nautilus)
+  etc/systemd/system/           System service (battery conservation restore on boot)
+  etc/udev/rules.d/             udev rules (battery threshold permissions)
   backgrounds/wallpaper.jpg     Default wallpaper (shared by desktop, greeter, lock screen)
 ```