Add sidebar with sprint goals

In sprint planning, you want to keep sprint goals in mind as you're allocating chips. (This is in addition to the hours allocation per project.) It'd be convenient to have a responsive sprint goal display:

- On large screens, it could be a floating sidebar
- On small screens, it could be a collapsible element triggered from a fixed-position footer element

Start by locating the sprint-planning interface and the existing hours-allocation UI. Add a responsive sprint-goal display that is a floating sidebar on large screens and a collapsible element opened from a fixed footer on small screens; done means goals remain visible while allocating chips at both sizes.

Written by the indexing model from the issue text.