Rework and finalize #2786: Unique phase‑specific URLs for “My Submissions,” “All Submissions,” and Leaderboard

### Description

Pull Request #2786 implements slug‑based URLs so each challenge phase under “My Submissions” and “All Submissions” has its own unique, shareable link. We also need to extend this to the Leaderboard view. Before merging, we must address outstanding review comments, fix build errors, update tests, and ensure API consistency (including slug support in analytics and leaderboard endpoints).