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/experiments/index.md

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+# Experiments
+
+记录可复现的模型、提示词、上下文、工具和工作流实验。
+
+每个实验至少包含假设、输入、对照、观察指标、结果和结论。不要把单次主观感受直接提升为 Pattern。