enhance(frontend): 押したリアクションのデザインを改善

[EbiseLutica]

What

resolve #11272

• 自分が押したリアクションのデザインを改善し、絵文字の視認性をあげました

Why

元issueを参照

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

Merging #11434 (5d29d10) into develop (61c9b07) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop   #11434      +/-   ##
===========================================
+ Coverage    78.75%   78.85%   +0.09%     
===========================================
  Files          921      920       -1     
  Lines        97886    97779     -107     
  Branches      7834     7863      +29     
===========================================
+ Hits         77093    77099       +6     
+ Misses       20793    20680     -113     

Files Changed	Coverage Δ
...tend/src/components/MkReactionsViewer.reaction.vue	100.00% <100.00%> (ø)

... and 18 files with indirect coverage changes

[syuilo]

👍