fix(localization): Update Japanese verification email send button label.

[dlr0w]

Description

This PR fixes the translation of "sendVerificationEmailLabel" in firebase_ui_ja.arb.
Currently, it is in English as "Send verification email", but it has been changed to "確認メールを送信" (Send verification email) in Japanese.
This will allow users to consistently see the Japanese display.

Related Issues

This change is only a localization fix and is not related to any specific issue.
If translation issues arise in the future, please refer to this PR.

Checklist

This PR fixes the translation of "sendVerificationEmailLabel" in firebase_ui_ja.arb.
Currently, it is in English as "Send verification email", but it has been changed to "確認メールを送信" (Send verification email) in Japanese.
This will allow users to consistently see the Japanese display.

• 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).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• All unit tests pass (melos run test:unit:all doesn't fail).
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

This change is only a localization fix and is not related to any specific issue.
If translation issues arise in the future, please refer to this PR.

• Yes, this is a breaking change.
• No, this is not a breaking change.

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