feat(ci): add typecheck and lint to PR workflow and fix existing errors

[Saloni3494]

What changed

• Added yarn typecheck and yarn lint steps to the .github/workflows/setup_and_build.yml workflow. Because this workflow is reused by both the test and deploy jobs, this enforces code quality checks on every PR and main branch push.
• Installed eslint-plugin-prettier as a dev dependency and upgraded prettier to resolve the module not found issue that was causing yarn lint to fail immediately on the main branch (addresses yarn lint fails immediately: ESLint couldn't find the plugin eslint-plugin-prettier #542).
• Fixed all 7 preexisting ESLint errors across the repository (e.g., removing unused imports, adding React map keys, and fixing broadly typed variables) so that the CI pipeline can successfully pass.

Why this change is needed

Currently, there is no typecheck or linting enforced in the CI pipeline for Pull Requests. This means typos, unused variables, and type mismatch errors can easily slip into the main branch unnoticed. By moving these checks directly into the GitHub Actions workflow, we establish a strict quality gate that prevents bad code from being deployed.

Because adding this check would have instantly broken the CI due to preexisting linting errors and missing dependencies on main, I also resolved the missing eslint-plugin-prettier module and patched the remaining ESLint errors throughout the codebase to ensure a green build.

Testing

• Verified yarn typecheck succeeds locally.
• Verified yarn lint succeeds locally with 0 errors.
• Verified YAML formatting in .github/workflows/setup_and_build.yml is valid.

Fixes: #607
Fixes: #542

Checklist

• Certify you wrote the patch or otherwise have the right to pass it on as an open-source patch by signing all commits. (git commit -s). The author email must match the sign-off email address.
• Referenced issues using Fixes: #00000 in the commit message.
• PR description, commit message, and GitHub comments are human-written, per the LLM Policy.