Standardize Node.js environment with .nvmrc

[Code-Paragon]

Problem

Different contributors may use varying Node.js versions, which could lead to inconsistent build outputs or incompatible dependency resolutions.

Expected behavior

An .nvmrc file specifies the exact Node version (v18+) to be used, aligning with the Next.js dashboard prerequisites.

Files to update

• .nvmrc (create)
• README.md

Project relevance

Removes setup friction for new contributors by instantly configuring their local node environments via NVM.

Acceptance criteria

• Feature works as described
• Tests are added or updated where needed
• Documentation is updated where needed
• The .nvmrc file contains v18 or 18
• No TypeScript, lint, test, or build errors