Challenge: Option to schedule a future start date for challenges

Currently, Habitify Challenges must be started manually, and the countdown begins immediately. It would be helpful to schedule a challenge to start on a specific future date - for example, setting up a “January 2026 Running Challenge”, challenge weeks ahead of time.

Feature proposal:

• Add a date picker when creating a challenge (“Start on…”).

• Allow participants to join before it starts.

• Start automatically at 00:00 on the chosen date, with notifications for all participants.

• Show countdown (“Challenge starts in 3 days”).

This would make planning easier for teams, groups, and individuals preparing for month‑long or new‑year challenges.