fix(frontend-embed): 型チェックエラーを修正

[samunohito]

What

（issue切ろうか迷ったけど冗長な気がしたのでそのままprしました）
frontend-embedのpnpm lintで検出できる型エラーを修正します。

※ #15215 の修正中に見つけたもので、同prが前提になります

Why

Additional info (optional)

• 型エラーが出なくなることを確認

Checklist

• Read the contribution guide
• Test working in a local environment
• (If needed) Add story of storybook
• (If needed) Update CHANGELOG.md
• (If possible) Add tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.69%. Comparing base (3c81926) to head (0cc74c6).
Report is 12 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #15216      +/-   ##
===========================================
+ Coverage    39.96%   41.69%   +1.73%     
===========================================
  Files         1563     1567       +4     
  Lines       197878   203758    +5880     
  Branches      3646     3733      +87     
===========================================
+ Hits         79080    84955    +5875     
- Misses      118193   118198       +5     
  Partials       605      605              

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