refactor(bar): modularize bar widgets and layout

modules/bar/LauncherButton.qml

@@ -0,0 +1,10 @@
+pragma ComponentBehavior: Bound
+
+import Quickshell
+import qs.ui
+
+BarButton {
+  iconOnly: true
+  iconSource: `${Quickshell.shellPath("assets")}/icons/nix.svg`
+  onClicked: Quickshell.execDetached(["vicinae", "toggle"])
+}