Projects: better applications, and team threads

Two Projects improvements:

Applications with substance. When you apply to a project role, you can now say exactly which of the role's skills you bring, how many hours a week you can commit (capped at 15 -- volunteer projects should fit around your life, not consume it), your working languages and timezone, and what you want to learn from the project. Your written pitch still matters; the structure just makes sure nothing important gets lost in prose.

Team threads on milestones. Once you're on a project team, every milestone has a private thread for coordinating with your teammates -- scoped to the project roster, attached to the actual work. This is deliberately not a general chat or DMs; it's a place to move a specific milestone forward.