Refactor power saving logic into display drivers and add ST7789 support

display/low_level_display.h

@@ -40,6 +40,11 @@ public:
     // Optional: Orientation control (not commonly needed for bitmap displays)
     virtual void set_rotation(uint8_t rotation) { (void)rotation; }
     
+    // Power saving hooks
+    virtual void on_idle_2min() {}
+    virtual void on_idle_10min() {}
+    virtual void on_user_interaction() {}
+
     // Factory method - creates display based on type, using board_config.h for pins
     static LowLevelDisplay* create(DisplayType type, int width, int height);
 };