change backlight in menu, not persistent

src/main.c

     debug("lcd_init");
     lcd_init();
     draw_splash();
-    lcd_set_backlight(0x8000);
+    lcd_set_backlight(0xFF00);
 
     if (watchdog_caused_reboot()) {
         debug("reset by watchdog");
 
     debug("init done");
     battery_run();
-    lcd_set_backlight(0x8000); // TODO dynamic
 
     // wait for BLE stack to be ready before using it
     debug("wait for bt stack");

src/menu.c

 #include "config.h"
 #include "buttons.h"
 #include "text.h"
+#include "lcd.h"
 #include "menu.h"
 
 static char prev_buff[MENU_MAX_LEN] = {0};
 
 static void menu_buttons(enum buttons btn, bool state) {
     if (state && (btn == BTN_LEFT)) {
-        // TODO brightness down
+        uint16_t backlight_value = lcd_get_backlight();
+        if (backlight_value > 0x00FF) {
+            backlight_value = backlight_value >> 1;
+        }
+        lcd_set_backlight(backlight_value);
         return;
     } else if (state && (btn == BTN_RIGHT)) {
-        // TODO brightness up
+        uint16_t backlight_value = lcd_get_backlight();
+        if (backlight_value < 0xFF00) {
+            backlight_value = backlight_value << 1;
+        }
+        lcd_set_backlight(backlight_value);
         return;
     } else if (state && ((btn == BTN_ENTER) || (btn == BTN_A))) {
         if (enter_callback) {