fix(ui_localizations): In the Japanese file, change "login" to "sign and improve translations

[ywake]

Description

• In the Japanese translation file, "Login" and "Sign in" are mixed, so I unified them with "Sign in" to match the English version.
  • There may be some debate as to whether to use "Login" or "Sign In" for this, but I decided to go with the English version.
• Removed unnatural spaces in Japanese.

• 日本語翻訳ファイルにおいて、「ログイン」と「サインイン」が混在しているので、英語版に合わせて「サインイン」に統一しました。
  • 「ログイン」と「サインイン」のどちらを使うべきかには議論があるかもしれませんが、英語版に合わせることにしました。
• 日本語の不自然な空白を削除しました。

Related Issues

none

Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [ ] My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• [x] All existing and new tests are passing.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] The analyzer (melos run analyze) does not report any problems on my PR.
• [x] All unit tests pass (melos run test:unit:all doesn't fail).
• [x] I read and followed the Flutter Style Guide.
• [x] I signed the CLA.
• [x] I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• [ ] Yes, this is a breaking change.
• [x] No, this is not a breaking change.

[russellwheatley]

Hey @ywake - is it possible you could get another developer fluent in Japanese to review this PR, please? I don't speak Japanese I'm afraid, but I am happy to get this PR merged providing it is an improvement 😄

[ywake]

@KoheiKanagu @keigomichi Hello, friends. I saw you in a past pull request, so I was wondering if you could review my pull request?

[ywake]

@russellwheatley I received a review 😄

[russellwheatley]

Thanks @KoheiKanagu & @ywake 🙏