docs: update README banner assets

[jikkai]

Summary

• Replace the separate light and dark README banners with the unified docs/img/banner.png asset.
• Update the root README and localized README pages to use the new banner and dimensions.
• Remove the obsolete banner-dark.png and banner-light.png assets.

Testing

• Not applicable: documentation and image asset changes only.
• Pre-commit eslint --fix completed successfully.

Checklist

• No breaking changes introduced.
• Related issue: none.

[github-actions]

Playwright test results

 12 passed

Details

 12 tests across 5 suites
 4 minutes, 41 seconds
 3726dc6
 For more information, see full report and artifacts

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.24%. Comparing base (ee1adb0) to head (ec2bfd0).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff            @@
##              dev    #7599    +/-   ##
========================================
  Coverage   81.24%   81.24%            
========================================
  Files        2715     2715            
  Lines      170524   170524            
  Branches    42361    42476   +115     
========================================
+ Hits       138536   138541     +5     
+ Misses      31940    31935     -5     
  Partials       48       48            

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