perf(frontend): Reduce getting clientWidth in MkMediaImage

[tamaina]

What

#11377 の続き

root.value.clientWidthが大変重たいため、スクロールコンテナごとにそれをキャッシュする

Why

パフォーマンス向上

Additional info (optional)

p1.a9z.devに適用

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

Merging #11386 (61d2bf0) into develop (545b5ca) will increase coverage by 0.03%.
Report is 2 commits behind head on develop.
The diff coverage is 58.62%.

@@             Coverage Diff             @@
##           develop   #11386      +/-   ##
===========================================
+ Coverage    77.68%   77.71%   +0.03%     
===========================================
  Files          918      920       +2     
  Lines        92601    92787     +186     
  Branches      7749     7749              
===========================================
+ Hits         71933    72107     +174     
- Misses       20668    20680      +12     

Files Changed	Coverage Δ
packages/frontend/src/components/MkMediaList.vue	96.69% <58.62%> (-3.31%)	⬇️

... and 2 files with indirect coverage changes

[syuilo]

👍🏻