fix: 启动页遮住登录/首页初始态，消除深色模式黑闪

- 新增 SplashPage + appReadyProvider/_BootGate：启动闸门将启动页覆盖在最上层，
  直到 auth 判定完成且今日健康卡数据就绪，遮住登录页闪现与首页对话流初始态
- auth 初始状态改为 isLoading=true，无 token 时再置 false
- values-night/styles.xml 改为亮色白底，消除深色模式下 Flutter 初始化期间的黑色窗口背景
- 带 8 秒安全兜底，避免离线时卡在启动页

health_app/android/app/src/main/res/values-night/styles.xml

@@ -1,18 +1,11 @@
 <?xml version="1.0" encoding="utf-8"?>
 <resources>
-    <!-- Theme applied to the Android Window while the process is starting when the OS's Dark Mode setting is on -->
-    <style name="LaunchTheme" parent="@android:style/Theme.Black.NoTitleBar">
-        <!-- Show a splash screen on the activity. Automatically removed when
-             the Flutter engine draws its first frame -->
+    <!-- 启动页强制亮色：即使系统处于深色模式，也用白底品牌图，避免黑色闪屏 -->
+    <style name="LaunchTheme" parent="@android:style/Theme.Light.NoTitleBar">
         <item name="android:windowBackground">@drawable/launch_background</item>
     </style>
-    <!-- Theme applied to the Android Window as soon as the process has started.
-         This theme determines the color of the Android Window while your
-         Flutter UI initializes, as well as behind your Flutter UI while its
-         running.
-
-         This Theme is only used starting with V2 of Flutter's Android embedding. -->
-    <style name="NormalTheme" parent="@android:style/Theme.Black.NoTitleBar">
-        <item name="android:windowBackground">?android:colorBackground</item>
+    <!-- Flutter 引擎初始化期间的窗口背景：白底，避免深色模式下闪黑 -->
+    <style name="NormalTheme" parent="@android:style/Theme.Light.NoTitleBar">
+        <item name="android:windowBackground">@android:color/white</item>
     </style>
 </resources>