feat(mobile): flutter scaffold with env script and baseline nav

- scripts/flutter-env.sh: toolchain PATH/JAVA_HOME/ANDROID_HOME + pub 镜像 env
- apps/mobile: flutter create (com.photolive.photo_live, android+ios)
  - Android minSdk 24 / targetSdk 36；INTERNET 权限（main manifest，debug+release）
  - iOS Info.plist：本地网络 ATS 例外 + 相册/相机用途说明（zh+en）
- 依赖接入：riverpod / drift(+sqlite3_flutter_libs) / dio / secure_storage /
  image_picker / camera / qr_flutter / path_provider / path / intl
- l10n：l10n.yaml + app_zh/app_en arb（20 键），gen-l10n（zh 模板，非空 getter）
- 基线：Material 3 主题、/login→/home 命名路由、底部导航（相册/队列/设置桩）、
  locale provider 脚手架（Task 10 完成切换）
- flutter analyze 零问题；widget 测试通过；debug apk 构建成功
  （MuMu 冒烟因宿主 commit 内存耗尽未完成，Task 2 重试）

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

apps/mobile/lib/app/app.dart

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+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../l10n/app_localizations.dart';
+import '../pages/home_page.dart';
+import '../pages/login_page.dart';
+import 'locale_controller.dart';
+import 'theme.dart';
+
+/// App 根组件：主题 + l10n 委托 + 基线路由。
+///
+/// 基线路由为命名路由（/login → /home）；Task 3+ 随页面增长再评估
+/// 是否迁移 go_router，当前保持零额外依赖。
+class PhotoLiveApp extends ConsumerWidget {
+  const PhotoLiveApp({super.key});
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final locale = ref.watch(localeSettingsProvider);
+    return MaterialApp(
+      onGenerateTitle: (context) => AppLocalizations.of(context).appName,
+      theme: AppTheme.light,
+      darkTheme: AppTheme.dark,
+      locale: locale,
+      supportedLocales: AppLocalizations.supportedLocales,
+      localizationsDelegates: AppLocalizations.localizationsDelegates,
+      routes: {
+        '/login': (context) => const LoginPage(),
+        '/home': (context) => const HomePage(),
+      },
+      home: const LoginPage(),
+    );
+  }
+}

apps/mobile/lib/app/locale_controller.dart

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+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+/// App 语言设置（脚手架）。
+///
+/// `null` = 跟随系统；Task 10 补齐：持久化（settings 存储）、
+/// 设置页交互与全部页面的 l10n 覆盖检查。
+class LocaleSettings extends Notifier<Locale?> {
+  @override
+  Locale? build() => null;
+
+  void setLocale(Locale? locale) => state = locale;
+}
+
+final localeSettingsProvider =
+    NotifierProvider<LocaleSettings, Locale?>(LocaleSettings.new);

apps/mobile/lib/app/theme.dart

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+import 'package:flutter/material.dart';
+
+/// 基础主题（Material 3，种子色）。完整视觉规范随后续任务迭代。
+abstract final class AppTheme {
+  static const Color seedColor = Color(0xFF1A73E8);
+
+  static ThemeData get light => ThemeData(
+        colorScheme: ColorScheme.fromSeed(seedColor: seedColor),
+      );
+
+  static ThemeData get dark => ThemeData(
+        colorScheme: ColorScheme.fromSeed(
+          seedColor: seedColor,
+          brightness: Brightness.dark,
+        ),
+      );
+}