fix z-index of left icons, to comply with the right ones

[ImTheKai]

Summary

• Restores a z-index fix that was made on a branch (z-index, commit baffc37) but never merged into main, so the popover hover box on topic-row icons has been rendering below/behind adjacent content — especially visible on Safari.
• A complementary fix (fixing the popover behavior, so it doesn't show as transparent, because later rows are in the front #54) landed separately around the same time (row gets z-index: 5 while a popover is open), but the matching bump on .topic-icon-hover itself and its container never made it in.

Restores:

• overflow: visible on .status-border
• z-index: 2 on .topic-title-icons
• .topic-icon-hover bumped from z-index: 20 to z-index: 50

Test plan

• Hover topic-list icons (left side) and confirm the popover renders in front of subsequent rows on Chrome/Windows and Safari/macOS