feat: A1 短材料复现本单元理解词，并加入黑夜模式

课程内容
- 重写全部 21 个 A1 单元的听读材料，把本单元理解词织进听/读文本，
  单元内理解词复现率从约 20% 提升到约 77%（各单元 56–96%）。
- 修正 U01 房间号与机场大巴同为 thirty 的撞车（改为 17/30/40）。
- 校验器容差按级别读取（A1 为 8%），materials 覆盖率、字数、
  选项子串等校验全部通过；course_content_test 通过。

黑夜模式
- app_theme 拆分明/暗两套调色板，AppColors 随亮度切换；
  main 用 theme/darkTheme/themeMode + builder 镜像已解析亮度；
  主题偏好持久化到快照；进度页新增“外观主题”切换。

文档
- COURSE-PACK-JSON.md 更新 A1 词池覆盖（840/933）与材料复现约定。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

kouyu_english/lib/main.dart

@@ -47,43 +47,60 @@ class _KouyuEnglishAppState extends State<KouyuEnglishApp> {
     super.dispose();
   }
 
+  static ThemeMode _themeMode(AppThemeMode mode) => switch (mode) {
+    AppThemeMode.system => ThemeMode.system,
+    AppThemeMode.light => ThemeMode.light,
+    AppThemeMode.dark => ThemeMode.dark,
+  };
+
   @override
   Widget build(BuildContext context) {
-    return MaterialApp(
-      title: '芽说英语 · SpeakSprout',
-      debugShowCheckedModeBanner: false,
-      theme: buildAppTheme(),
-      home: AnimatedBuilder(
-        animation: appState,
-        builder: (context, _) {
-          if (!appState.isLoaded) {
-            return const Scaffold(
-              body: Center(child: CircularProgressIndicator()),
-            );
-          }
-          if (appState.onboardingComplete) {
-            return LearningShell(state: appState);
-          }
-          if (onboardingStep == 0) {
-            return WelcomePage(
-              state: appState,
-              onContinue: () => setState(() => onboardingStep = 1),
-            );
-          }
-          return PopScope(
-            canPop: false,
-            onPopInvokedWithResult: (didPop, _) {
-              if (!didPop) {
-                setState(() => onboardingStep = 0);
-              }
-            },
-            child: PlacementPage(
-              state: appState,
-              onBack: () => setState(() => onboardingStep = 0),
-              onStart: () => appState.finishOnboarding(),
-            ),
-          );
+    return AnimatedBuilder(
+      animation: appState,
+      builder: (context, _) => MaterialApp(
+        title: '芽说英语 · SpeakSprout',
+        debugShowCheckedModeBanner: false,
+        theme: buildAppTheme(Brightness.light),
+        darkTheme: buildAppTheme(Brightness.dark),
+        themeMode: _themeMode(appState.themeMode),
+        // Mirror the resolved brightness into the global palette that many
+        // widgets read directly, so it tracks both the user's choice and any
+        // system dark-mode change.
+        builder: (context, child) {
+          AppColors.applyBrightness(Theme.of(context).brightness);
+          return child ?? const SizedBox.shrink();
         },
+        home: Builder(
+          builder: (context) {
+            if (!appState.isLoaded) {
+              return const Scaffold(
+                body: Center(child: CircularProgressIndicator()),
+              );
+            }
+            if (appState.onboardingComplete) {
+              return LearningShell(state: appState);
+            }
+            if (onboardingStep == 0) {
+              return WelcomePage(
+                state: appState,
+                onContinue: () => setState(() => onboardingStep = 1),
+              );
+            }
+            return PopScope(
+              canPop: false,
+              onPopInvokedWithResult: (didPop, _) {
+                if (!didPop) {
+                  setState(() => onboardingStep = 0);
+                }
+              },
+              child: PlacementPage(
+                state: appState,
+                onBack: () => setState(() => onboardingStep = 0),
+                onStart: () => appState.finishOnboarding(),
+              ),
+            );
+          },
+        ),
       ),
     );
   }