fix: 🐛 update autocomplete default width

[xxholly32]

✅ Closes: 16619

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[pull-request-triage]

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[github-actions]

[github-actions]

Hello @xxholly32, thank you for contributing to element-plus, please see our guideline to see how to make contribution

[github-actions]

🧪 Playground Preview: https://element-plus.run/?pr=16633
Please comment the example via this playground if needed.

[kooriookami]

Use getCssVar('input-width') is better.

[xxholly32]

@kooriookami see if its OK, ty