feat: initial release

Fixed volume steps for Bluetooth audio devices on PipeWire. Corrects the
coarse internal volume grid of devices such as AirPods Pro (16 AVRCP steps,
6.25 % per swipe) to the desktop's own step size.

The device grid is measured per device; the desktop step size is read from
Plasma. English and German messages.

systemd/bt-volume-step.service

@@ -0,0 +1,14 @@
+[Unit]
+Description=Fixed volume steps for Bluetooth audio devices
+Documentation=https://github.com/Felitendo/bt-volume-step
+After=pipewire-pulse.service
+Wants=pipewire-pulse.service
+
+[Service]
+Type=simple
+ExecStart=/usr/bin/bt-volume-step
+Restart=always
+RestartSec=3
+
+[Install]
+WantedBy=default.target