fix: update deprecated codemirror import to restore Storybook build

[rohith-1205]

Summary

Storybook’s CSS build was failing because Sass no longer supports the ~ tilde import syntax.

Problem

The file client/styles/main.scss still uses the old pattern:
@import '~codemirror/lib/codemirror';

This causes Sass to throw:
"Can't find stylesheet to import"

Fix

Updated the SCSS import to the modern path:
@import 'codemirror/lib/codemirror.css';

This resolves the Storybook build issue and improves contributor experience.

No breaking changes.

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[rohith-1205]

Here is Storybook running after applying the SCSS import fix.
(Note: The ThemeProvider issue is unrelated and exists on main branch too.)