feat: complete kouyu_english app codebase, A0 specifications and .gitignore

kouyu_english/lib/main.dart

@@ -0,0 +1,66 @@
+import 'package:flutter/material.dart';
+
+import 'core/app_state.dart';
+import 'core/app_theme.dart';
+import 'features/onboarding/onboarding_pages.dart';
+import 'features/shell/learning_shell.dart';
+
+void main() {
+  runApp(const KouyuEnglishApp());
+}
+
+class KouyuEnglishApp extends StatefulWidget {
+  const KouyuEnglishApp({super.key});
+
+  @override
+  State<KouyuEnglishApp> createState() => _KouyuEnglishAppState();
+}
+
+class _KouyuEnglishAppState extends State<KouyuEnglishApp> {
+  final appState = AppState();
+  var onboardingStep = 0;
+
+  @override
+  void initState() {
+    super.initState();
+    appState.load();
+  }
+
+  @override
+  void dispose() {
+    appState.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: '开口英语',
+      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 PlacementPage(
+            state: appState,
+            onStart: () => appState.finishOnboarding(),
+          );
+        },
+      ),
+    );
+  }
+}