[2.x] fix: add left padding to posts on mobile

[imorland]

Summary

• Fixes uneven post padding on mobile devices (issue [2.x] Mobile Post Formatting #4415)
• .Post base style has padding-left: 0 since the avatar grid column provides implicit left spacing on desktop/tablet; on mobile the grid and avatar are removed, leaving content flush against the left edge
• Adds padding-left: var(--post-padding) inside @media @phone so both sides have symmetrical 20px padding

Test plan

• Open a discussion on a mobile device or browser devtools at ≤767px viewport width
• Verify posts have equal left and right padding on both comment posts and event posts
• Verify event post icons still align correctly
• Verify thread divider lines between posts still render correctly

Closes #4415

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