Challenges: add difficulty filter using StateDifficulty

[michaelsimeon001]

Add a filter control (leveraging the existing StateDifficulty component) so users can filter challenges by difficulty level.

Acceptance criteria

• Difficulty filter control added to the challenges view
• Challenge list filters correctly by selected difficulty

[AbdulmujibOladayo]

Hi! I'd like to work on this issue. Could you please assign it to me? Thanks!