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/learnings/index.md

@@ -0,0 +1,14 @@
+# Learnings
+
+保存已有明确证据、适用边界和首选行动的经验。
+
+按实际增长情况建立主题目录，例如：
+
+- debugging
+- planning
+- testing
+- review
+- context-engineering
+- agent-workflows
+
+不要预先创建空分类。