Fix reading cover upload layout

[castroofelipee]

Summary

• prevent the reading cover upload control from shrinking inside the drawer's flex form
• preserve the intended 2:3 cover area with a 13.5rem minimum height
• restore the visible upload target instead of rendering it as a thin clickable line

Validation

• git diff --check passes
• npm run lint remains blocked by pre-existing react-hooks/set-state-in-effect errors in Boards and Mail
• npm run build reached the optimized production build step but did not complete and was interrupted