Fix: Fix signin problem on Vercel Preview environment

[nick-y-ito]

Overview

This PR fixes issue #338, which causes sign-in failure.

Changes

• Created scripts/setupEnv.mjs
• Modified the postinstall script in package.json

Review points

• I have confirmed that the creation of .env.development.local file was skipped correctly on my Vercel account.

Screen Captures

References

• Vercel - System Environment Variables Overview
• Vercel - Environment Variable Load Order
• Vercel - Environment Variable Load Order

Environment Variable Load Order

Environment variables are looked up in the following places, in order, stopping once the variable is found.

1. process.env
2. .env.$(NODE_ENV).local
3. .env.local (Not checked when NODE_ENV is test.)
4. .env.$(NODE_ENV)
5. .env

For example, if NODE_ENV is development and you define a variable in both .env.development.local and .env, the value in .env.development.local will be used.

Notes

• This PR closes #326

Assignee Checklist:

• [x] The naming convention of the PR title is correct (See the comment at the top of this template)
• [x] The base branch is correct (See: Types of Branches)
• [x] The branch name follows the Branch Naming Conventions
• [x] The correct assignees and reviewers have been designated for this PR
• [x] The coding style follows the Coding Style Guide
• [x] All the related issues are associated with this PR
• [x] All criteria in the associated issues are met (please tick the checkboxes)
• [x] My changes do not generate new warnings or errors (especially in the console)

Reviewer Checklist:

• [ ] Double-check the "Assignee Checklist"
• [ ] The code follows the generic best practices and our Coding Style Guide
• [ ] The code is well-commented and easy to understand
• [ ] The UI changes accurately reflect the provided design

[nick-y-ito]

@kanta1207 @ShoeheyOt @kotaaaa Could you please review this PR when you have a moment?

[nick-y-ito]

@kotaaaa Thank you for your review!