[2.x] fix: reduce post horizontal padding on mobile to 15px

[imorland]

Summary

Follow-up to #4422.

The fix in #4422 added padding-left: var(--post-padding) (20px) to match the existing right padding on mobile, but 20px on both sides feels excessive on small screens.

This reduces both sides to 15px on mobile, which:

• Matches the existing PostStream-item horizontal padding pattern on mobile (padding: 0 15px)
• Gives a consistent, symmetric 15px on left and right
• Frees up a little more content width on small screens

Test plan

• Verify posts have symmetric 15px left and right padding on mobile (≤767px)
• Verify event posts, dividers, and badges still render correctly