docs: update repository directory structure

[ChestnutForest]

Summary

• update the README directory tree to match the repository's current tracked structure
• document the implemented backend and frontend subdirectories
• list the current requirements analysis, decision, architecture, devlog, and history documents
• clarify that generated and Git-ignored files are omitted

Validation

• compared the feature branch with main
• confirmed the branch is one commit ahead and not behind
• confirmed README.md is the only changed file
• verified the directory section and following heading boundaries
• verified the newly documented paths against the GitHub repository tree