feat(app): 初始化 Flutter 前端项目 (Phase F0)

- Flutter 3.44.0 + Dart 3.12.0
- 设计系统: 7色Token×浅/深模式, NotoSansSC/Caveat字体, 圆角10/16/22/28/pill, 三级阴影
- ResponsiveScaffold: 手机底部TabBar / 平板侧边Rail / 桌面三栏
- go_router 路由表: 13个页面 (5个Tab + 8个全屏页面)
- 13个功能模块占位页面 (home/calendar/mood/search/profile/editor/auth/class/teacher/parent/achievement/stickers/templates)
- 依赖: flutter_bloc, go_router, freezed, isar, dio, perfect_freehand, fl_chart
- 中国镜像: PUB_HOSTED_URL + FLUTTER_STORAGE_BASE_URL
- flutter analyze: No issues found

app/analysis_options.yaml

@@ -0,0 +1,28 @@
+# This file configures the analyzer, which statically analyzes Dart code to
+# check for errors, warnings, and lints.
+#
+# The issues identified by the analyzer are surfaced in the UI of Dart-enabled
+# IDEs (https://dart.dev/tools#ides-and-editors). The analyzer can also be
+# invoked from the command line by running `flutter analyze`.
+
+# The following line activates a set of recommended lints for Flutter apps,
+# packages, and plugins designed to encourage good coding practices.
+include: package:flutter_lints/flutter.yaml
+
+linter:
+  # The lint rules applied to this project can be customized in the
+  # section below to disable rules from the `package:flutter_lints/flutter.yaml`
+  # included above or to enable additional rules. A list of all available lints
+  # and their documentation is published at https://dart.dev/lints.
+  #
+  # Instead of disabling a lint rule for the entire project in the
+  # section below, it can also be suppressed for a single line of code
+  # or a specific dart file by using the `// ignore: name_of_lint` and
+  # `// ignore_for_file: name_of_lint` syntax on the line or in the file
+  # producing the lint.
+  rules:
+    # avoid_print: false  # Uncomment to disable the `avoid_print` rule
+    # prefer_single_quotes: true  # Uncomment to enable the `prefer_single_quotes` rule
+
+# Additional information about this file can be found at
+# https://dart.dev/guides/language/analysis-options