docs(contributing): rewrite contributor guide for current repo

Replace the obsolete Python/ComfyUI-era CONTRIBUTING guide with a repo-accurate contribution document for the current Rust, React, Electron, and launcher codebase.

Document what good contributions look like, the current toolchain pins, canonical setup and launcher workflows, repository boundaries, planning expectations, validation requirements, documentation duties, and commit/PR expectations so new contributors can work from one practical entrypoint.

Update frontend README links that referenced the old CONTRIBUTING contract so they now point at the rewritten root guide and describe it as contribution, error-handling, and validation guidance instead of frontend-only standards.

Author: MrScripty