Use light sleep to save battery in tracker

src/main.cpp

@@ -350,6 +350,7 @@ public:
         }
 
         M5.Display.endWrite();
+        sleep();
     }
 
     App::Choices loop () {
@@ -387,9 +388,25 @@ public:
             next_clock_update = next_minute(current_time);
             drawClock(current_time);
             drawBattery();
+        } else {
+            sleep();
         }
         return App::LifeTracker;
     }
+
+    void sleep() {
+        const time_t current_time = time(nullptr);
+        if (current_time >= next_clock_update) return;
+
+        int seconds = next_clock_update - current_time;
+        esp_sleep_enable_ext0_wakeup(GPIO_NUM_36, LOW); // Wake on touch
+        esp_sleep_enable_ext0_wakeup(GPIO_NUM_37, LOW); // Wake on button up
+        esp_sleep_enable_ext0_wakeup(GPIO_NUM_38, LOW); // Wake on button middle
+        esp_sleep_enable_ext0_wakeup(GPIO_NUM_39, LOW); // Wake on button down
+        esp_sleep_enable_timer_wakeup(seconds * 1000); // Wake up at the next clock update (in microseconds)
+        esp_light_sleep_start();
+    }
+
     void shutdown() {
         for (int i = 0; i < 4; i++) {
             delete players[i];