Challenge: Public challenge directory and "publish to everyone" option

It would be great to bring back the sense of community from the old Habitify Challenges, where anyone could see and join challenges from other users or Habitify Team. Right now, challenges are invite-only, which makes participation limited if you don’t personally know many users.

Requested features:

• Public challenge directory: We should be able to browse, search, and join public challenges made by other users.

• “Publish to everyone” option: When creating a challenge, allow users to make it visible to the entire Habitify community.

• Tags and filters: By category, language, difficulty, or type (e.g. health, productivity). This could help if there are a lot of challenges posted.

This would restore the motivational aspect of seeing thousands of people doing the same challenge together and could also attract new users to try Habitify.