feat(ui): add responsive styling for mobile compatibility

[liamsegura]

Fixes https://github.com/decaporg/decap-cms/issues/7241

Summary

Long time user of decap-cms, always wanted a way to use while on my phone, this PR fixes that.

Added media queries for responsive styling <meta name="viewport" content="width=device-width, initial-scale=1.0" /> is required in the html for it to work, i have only updated the dev-test html

If this is merged in I can go through the other repos and update that accordingly

I can improve this visually as there are areas id like to modify but that can be altered at a later date

https://github.com/user-attachments/assets/5a80957c-e81e-4966-8f78-94be6cf17148

Test plan

Checklist

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