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().

App/main.c

@@ -257,7 +257,7 @@ void main(void)
     RGB_Send(0, 0, 0, 0, 0, 0); // 【首要修正】：上电第一时间强制发送 0 码关闭灯条，熄灭上电暂态引起的随机白光！
     RF_Init();         // 初始化射频芯片各引脚控制
     Delay_ms(500);     // 软启动延时，等待供电电容稳定
-    LCD_Init();        // 初始化 LHS114TC-IF03 (ST7789V) 屏幕控制寄存器序列并开启背光
+    LCD_Init();        // 初始化 LHS114TC-IF03 (ST7789V) 屏幕控制寄存器序列 (背光保持关闭，避免闪烁)
     Uart1_Init();      // 初始化串口 1 波特率 115200 供调试日志
     Uart_SendString("Wristband System Initialized!\r\n");
 
@@ -281,6 +281,9 @@ void main(void)
 
     Uart_SendString("[SYS] Event-driven framework ready\r\n");
 
+    // 所有初始化与首帧时钟界面绘制均已完成，此刻才点亮背光，避免开机过程中的中间画面被看到而产生闪烁
+    SGM_CTRL = 0;
+
     // ====== 主循环 ======
     while (1) {
         /* 0. 检查主循环按键定时标志 (由 1ms 中断置位，主循环响应清除并处理) */

App/system.c

@@ -60,6 +60,9 @@ void GPIO_Init(void)
     // SGM_CTRL (P1.7), LCD 控制管脚 (P1.0, P1.1, P1.4, P1.5, P1.6) 置为推挽输出
     P1M1 &= ~((1 << 7) | (1 << 0) | (1 << 1) | (1 << 4) | (1 << 5) | (1 << 6));
     P1M0 |= ((1 << 7) | (1 << 0) | (1 << 1) | (1 << 4) | (1 << 5) | (1 << 6));
+    SGM_CTRL = 1; // 背光从上电第一刻起就确定关闭 (LEDA=1)，避免推挽模式生效前电平不确定造成瞬间闪烁
+
+
 
     // SHUT (P3.5) 射频/外设开启使能脚置为推挽输出
     P3M1 &= ~(1 << 5);