Improve styles of Now view

Follow up to #1237

• [ ] Animate buttons when a task is completed or a new one is focused
• [ ] Add back arrow button in top left
• [ ] Make focus color the same color as when focusing on a task/agenda event on the day view.
• [ ] Replace hard-coded colors: https://github.com/SwitchbackTech/compass/pull/1266#discussion_r2532212433
• [ ] Fix unresolved comments here: https://github.com/SwitchbackTech/compass/pull/1266
• [x] Update shortcuts overlay
• [x] add tooltip when hovering over now buttons

<!-- upstream-issue: SwitchbackTech/compass#1244 -->