refactor(bar): restructure panel window and corners

Battery.qml

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+pragma Singleton
+
+import Quickshell
+import QtQuick
+import Quickshell.Services.UPower
+
+Singleton {
+    id: root
+    readonly property int percentage: Math.round(UPower.displayDevice.percentage * 100)
+    readonly property real energyRate: UPower.displayDevice.changeRate
+    readonly property string iconName: root.percentage < 30 ? "battery-low.svg" : root.percentage < 70 ? "battery-medium.svg" : "battery-full.svg"
+    readonly property url icon: `${Quickshell.shellPath("assets")}/icons/${root.iconName}`
+}