fix(boot): hold backlight off until first frame is drawn

SGM_CTRL was turned on at the very start of LCD_Init(), before the
ST7789V register sequence and first frame ran, showing garbage/blank
content briefly on every boot. GPIO_Init() now forces backlight off at
power-on, LCD_Init() no longer touches it, and main() only turns it on
right before entering the main loop (after HomeApp's first frame is
drawn). Wakeup_Restore()'s existing on-then-reinit sequence is
untouched since it sets the level itself before calling LCD_Init().

Drivers/lcd.c

@@ -85,8 +85,9 @@ void LCD_Init(void)
     P1M1 &= ~((1<<0) | (1<<1) | (1<<4) | (1<<5) | (1<<6) | (1<<7));
     P1M0 |=  ((1<<0) | (1<<1) | (1<<4) | (1<<5) | (1<<6) | (1<<7));
 
-    // 设置各控制引脚初始默认电平
-    SGM_CTRL = 0; // PMOS低电平点亮背光 (LEDA = 0)
+    // 设置各控制引脚初始默认电平 (LEDA 背光电平不在这里设置——开机路径由 GPIO_Init() 提前
+    // 关闭、main() 即将进入主循环前才点亮；休眠唤醒路径由 Wakeup_Restore() 在调用本函数前
+    // 先行点亮；两条路径对背光的时序要求不同，LCD_Init() 本身不做任何背光电平的假设)
     LCD_CS = 1;
     LCD_SCL = 1;
     LCD_RST = 1;