fix(frontend): 投稿フォームでのユーザー変更がプレビューに反映されるように修正

[zyoshoka]

What

投稿フォームでユーザーを変更したときにプレビューに反映されるようにしました

Why

Fix: #11899

Additional info (optional)

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 ✅

Comparison is base (2697826) 79.36% compared to head (7d0af31) 79.37%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #12022      +/-   ##
===========================================
+ Coverage    79.36%   79.37%   +0.01%     
===========================================
  Files          939      941       +2     
  Lines       100332   100503     +171     
  Branches      8183     8175       -8     
===========================================
+ Hits         79625    79779     +154     
- Misses       20707    20724      +17     

Files	Coverage Δ
packages/frontend/src/components/MkNotePreview.vue	100.00% <100.00%> (ø)
packages/frontend/src/components/MkPostForm.vue	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes

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[syuilo]

🙏