Stick to npm as the single app package manager

[jddark62]

Closes #20.

Summary

• Removed app/yarn.lock so the app no longer carries both npm and Yarn lockfiles.
• Kept the existing npm workflow intact because install.sh and the package scripts already use npm install, npm start, and npm run build.
• Added a short README note telling contributors to keep app/package-lock.json as the single lockfile.

Verification

• git diff --check
• Confirmed app/yarn.lock is removed.
• Parsed app/package.json and app/package-lock.json successfully with Node.js.
• Searched README, package scripts, and install script for yarn.lock, package-lock, and npm references.

I could not run npm install or the app build in this workspace because npm/yarn/pnpm/npx are not installed here. The change is limited to removing the unused competing lockfile and documenting the package-manager choice.

[Copilot]

Pull request overview

This pull request standardizes the frontend app’s package-management approach by committing to npm as the single package manager and documenting that choice for contributors, addressing Issue #20.

Changes:

• Removed the competing Yarn lockfile (app/yarn.lock) so the app isn’t tracked by two lockfiles.
• Documented that app/package-lock.json should remain the single lockfile and contributors should not regenerate app/yarn.lock.

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