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/.metadata

@@ -0,0 +1,33 @@
+# This file tracks properties of this Flutter project.
+# Used by Flutter tool to assess capabilities and perform upgrades etc.
+#
+# This file should be version controlled and should not be manually edited.
+
+version:
+  revision: "559ffa3f75e7402d65a8def9c28389a9b2e6fe42"
+  channel: "stable"
+
+project_type: app
+
+# Tracks metadata for the flutter migrate command
+migration:
+  platforms:
+    - platform: root
+      create_revision: 559ffa3f75e7402d65a8def9c28389a9b2e6fe42
+      base_revision: 559ffa3f75e7402d65a8def9c28389a9b2e6fe42
+    - platform: android
+      create_revision: 559ffa3f75e7402d65a8def9c28389a9b2e6fe42
+      base_revision: 559ffa3f75e7402d65a8def9c28389a9b2e6fe42
+    - platform: ios
+      create_revision: 559ffa3f75e7402d65a8def9c28389a9b2e6fe42
+      base_revision: 559ffa3f75e7402d65a8def9c28389a9b2e6fe42
+
+  # User provided section
+
+  # List of Local paths (relative to this file) that should be
+  # ignored by the migrate tool.
+  #
+  # Files that are not part of the templates will be ignored by default.
+  unmanaged_files:
+    - 'lib/main.dart'
+    - 'ios/Runner.xcodeproj/project.pbxproj'