fix: 投稿フォームの字数上限計算を実際の投稿内容に合わせる

[KisaragiEffective]

What

close #14410

Why

issueに記載のため省略

Additional info (optional)

Checklist

• [x] 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

[syuilo]

🙏🏻

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 20.21%. Comparing base (44f6216) to head (366602f). Report is 1 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #14466 +/- ## ======================================== Coverage 20.21% 20.21% ======================================== Files 725 725 Lines 100550 100550 Branches 1050 1050 ======================================== Hits 20327 20327 Misses 79681 79681 Partials 542 542 ```

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