Fix table of contents heading indentation

What does this MR do and why?

Contributes to #627811 (closed)

Problem

Vue 3 compat forwards TOC extraAttrs through a legacy object-bind helper that treats inline styles as classes. This turns heading padding into invalid CSS and breaks indentation in the file preview TOC.

Solution

• Replace dynamic padding styles with GitLab Tailwind padding classes.
• Preserve the 16px through 56px indentation sequence with !important overrides for GitLab UI item padding.
• Verify rendered classes with real GlDisclosureDropdown, including deep heading levels.

This is a Vue 3 compat regression. The compat flag that introduced the problem has since been removed, so rolling it back is not available.

References

#627811 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create a project with README
2. Check TOC rendering (using Vue 3)
3. Confirm that the indentation is correct

MR acceptance checklist

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