enhance(frontend): テーマ設定で簡易プレビューを表示するように

[kakkokari-gtyih]

What

Why

UI改善
https://misskey.io/notes/a5629k4noswh0d0b

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

Attention: Patch coverage is 0% with 208 lines in your changes missing coverage. Please review.

Project coverage is 15.35%. Comparing base (9dce512) to head (adc0ce8).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
packages/frontend/src/pages/settings/theme.vue	0.00%	136 Missing ⚠️
...ackages/frontend/src/components/MkThemePreview.vue	0.00%	68 Missing and 1 partial ⚠️
...ntend/src/utility/autogen/settings-search-index.ts	0.00%	2 Missing ⚠️
packages/frontend/src/theme.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop   #15643       +/-   ##
============================================
- Coverage    41.92%   15.35%   -26.57%     
============================================
  Files         1624      787      -837     
  Lines       165871    72311    -93560     
  Branches      4106     1279     -2827     
============================================
- Hits         69540    11103    -58437     
+ Misses       95854    60783    -35071     
+ Partials       477      425       -52     

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[mi-gh-maintainer]

Thank you 🙏