feat: add brightness slider to Waybar backlight module

Wrap backlight in a drawer group with backlight/slider, mirroring the
existing sound group pattern. Slider appears on hover/click.

defaults/xdg/waybar/style.css

@@ -66,6 +66,7 @@ widget widget>* {
 #sys,
 #stats,
 #sound,
+#brightness,
 #indicators {
     padding: 0 4px;
     color: alpha(@theme_text_color, 0.5);
@@ -114,6 +115,36 @@ widget widget>* {
     color: #cc3436;
 }
 
+/* --- Brightness --- */
+
+#backlight-slider {
+    padding-left: 10px;
+    padding-right: 10px;
+}
+
+#backlight-slider slider {
+    min-height: 0px;
+    min-width: 0px;
+    opacity: 0;
+    background-image: none;
+    border: none;
+    box-shadow: none;
+    background-color: @insensitive_bg_color;
+}
+
+#backlight-slider trough {
+    min-height: 4px;
+    min-width: 80px;
+    border-radius: 5px;
+    background-color: alpha(@theme_selected_bg_color, 0.3);
+}
+
+#backlight-slider highlight {
+    min-width: 5px;
+    border-radius: 5px;
+    background-color: alpha(@theme_selected_bg_color, 0.5);
+}
+
 /* --- Battery group (hidden when empty, requires Waybar PR #4941) --- */
 
 #bat.empty,