docs: standardize feature README sections across demos

[however-yir]

Summary

Standardize the frontend demo README into a project-specific structure.

Why

The current file is still the default Vite template, which means readers do not get project-specific setup guidance where they expect it. A short README with clear sections for overview, setup, run, and expected behavior is more useful for contributors.

Scope

• rewrite front-end/README.md into a project-specific guide
• standardize the sections around overview, prerequisites, setup, run, and expected behavior
• keep the document concise and focused on the current frontend workflow

Testing

• documentation-only changes
• reviewed the commands and config references against the current frontend package scripts and env files