feat: add frontend development guidelines and structure documentation

- Introduced API guidelines for interface contracts and request handling.
- Added design tokens usage guidelines for consistent styling across the project.
- Established DTO guidelines for defining request parameters and response data types.
- Created frontend structure guidelines to clarify directory organization and code placement rules.
- Compiled a comprehensive frontend development guideline document covering various aspects of the development process.
- Implemented quality guidelines to ensure code maintainability and adherence to best practices.

AI Coding/inbox/index.md

@@ -0,0 +1,7 @@
+# Inbox
+
+保存刚从任务、用户纠正、失败路径或成功实践中提取的候选经验。
+
+候选经验尚不能直接视为全局规则。验证后移动到 `learnings/`；没有复用价值的内容直接删除。
+
+文件命名：`YYYYMMDD-short-slug.md`。