chore(design): 删除 design/flutter fork，更新 .gitignore

删除 design/flutter/（21 个文件）：
- Flutter token/组件的 canonical 实现已在 client/lib/
- Token 数值层改由 design/codegen/gen_flutter_tokens.mjs codegen
- 设计规格由 design/preview/ specimens + SKILL.md 表达

.gitignore 新增：
- client/android/local.properties、client/android/app/src/main/java/
- client/ios/Flutter/ephemeral/、client/pubspec.lock
- .claude/worktrees/（临时工作目录）

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

.gitignore

@@ -49,6 +49,15 @@ app/kernel/.build/
 server/mockserver
 server/pangolin-server
 
+# Flutter / Dart 构建产物与本地配置
+client/android/local.properties
+client/android/app/src/main/java/
+client/ios/Flutter/ephemeral/
+client/pubspec.lock
+
+# Claude worktrees（临时工作目录，本地专用）
+.claude/worktrees/
+
 # 杂项
 .DS_Store
 *.log