fix(ListItemIcon): use theme.spacing instead of hardcoded minWidth

[adiitxa]

Summary

This PR replaces the hardcoded minWidth: 56px in ListItemIcon with theme.spacing(4.5) (36px), aligning spacing with other components like MenuItem. This ensures consistent theming and avoids layout inconsistencies.

This issue is visible in the MUI here:
👉 https://mui.com/material-ui/react-list/#nested-list
(Nested ListItemIcon shows inconsistent spacing due to hardcoded minWidth)

Fixes

Closes #46596
Ensures icon spacing is responsive and theme-consistent.

Testing

• Verified visually in local Storybook and MUI docs (List, Menu)
• Confirmed spacing updates across RTL and default themes

Screenshots

🔴 Before

✅ After

Checklist

• Follows MUI theme conventions
• No hardcoded styles
• Visual regression verified
• Added screenshot proof

[mui-bot]

Netlify deploy preview

https://deploy-preview-46597--material-ui.netlify.app/

Bundle size report

Bundle	Parsed Size	Gzip Size
@mui/material	🔺+12B(0.00%)	🔺+7B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

Generated by 🚫 dangerJS against c91f6e4

[adiitxa]

Hi! This PR fixes #46596
Could a maintainer please add the appropriate test label (test:skip or test:done) to pass the required check? This fix is purely visual
could you please add a test label here? 🙏

Thanks!

[adiitxa]

Hi @ZeeshanTamboli
PR aligns ListItemIcon spacing with theme.spacing(4.5). Verified with screenshots.

review when you get a chance. Thanks! 🙏

[siriwatknp]

Thanks for submitting the PR. It's definitely better to me but it's a breaking change so I don't think this can be slot in v7. I'll keep this open.