fix(ui): shorten list difficulty to E/M/H

[tomtom-bot7]

Summary

• Problem list difficulty column shows E / M / H instead of Easy / Medium / Hard.
• Narrows the column so titles truncate less; full name still available via title / aria-label.
• Filter menu labels stay Easy / Medium / Hard.

Test plan

• Open the problem list: difficulty shows as E/M/H with the same colors.
• Hover a letter: tooltip shows Easy/Medium/Hard.
• Titles have more room before truncating.
• Difficulty filter chips still use full words.