feat: add laptop/display brightness modulation

programs/include/gui/desktop.hpp

@@ -227,6 +227,18 @@ struct DesktopState {
     montauk::abi::BatteryInfo battery_info;
     uint64_t battery_last_poll;
 
+    // Display brightness.  Present only while the active output has a
+    // controllable backlight (laptop panel, or a monitor answering DDC/CI).
+    SvgIcon icon_brightness;
+    bool brightness_present;
+    bool brightness_popup_open;
+    Rect brightness_icon_rect;
+    int brightness_level;                  // 0-100, as shown on the slider
+    bool brightness_dragging;
+    int brightness_pending;                // drag sample not yet sent, -1 none
+    uint64_t brightness_last_send;
+    uint64_t brightness_last_poll;
+
     bool vol_popup_open;
     Rect vol_icon_rect;
     int vol_level;            // 0-100