extract BacklightService singleton, deduplicate brightness logic

shell/services/BacklightService.qml

@@ -0,0 +1,64 @@
+pragma Singleton
+
+import QtQuick
+import Quickshell
+import Quickshell.Io
+import "." as S
+
+QtObject {
+    id: root
+
+    property int percent: 0
+    readonly property bool available: _blDev !== ""
+
+    function adjust(delta) {
+        const step = S.Modules.backlight.step || 5;
+        _adjProc.cmd = delta > 0 ? "light -A " + step : "light -U " + step;
+        _adjProc.running = true;
+    }
+
+    function setPercent(pct) {
+        _adjProc.cmd = "light -S " + Math.round(Math.max(0, Math.min(100, pct)));
+        _adjProc.running = true;
+    }
+
+    property string _blDev: ""
+
+    property Process _detectProc: Process {
+        running: true
+        command: ["sh", "-c", "ls /sys/class/backlight/ 2>/dev/null | head -1"]
+        stdout: StdioCollector {
+            onStreamFinished: {
+                const dev = text.trim();
+                if (dev)
+                    root._blDev = "/sys/class/backlight/" + dev;
+            }
+        }
+    }
+
+    property FileView _current: FileView {
+        path: root._blDev ? root._blDev + "/brightness" : ""
+        watchChanges: true
+        onFileChanged: reload()
+        onLoaded: root._update()
+    }
+
+    property FileView _max: FileView {
+        path: root._blDev ? root._blDev + "/max_brightness" : ""
+        onLoaded: root._update()
+    }
+
+    function _update() {
+        const c = parseInt(_current.text());
+        const m = parseInt(_max.text());
+        if (m > 0)
+            percent = Math.round((c / m) * 100);
+    }
+
+    property Process _adjProc: Process {
+        property string cmd: ""
+        command: ["sh", "-c", cmd]
+        onRunningChanged: if (!running && cmd !== "")
+            root._current.reload()
+    }
+}