Keep task group headers visible while scrolling

[christenbc]

Summary

Grouped task lists are easier to scan when you can always see which section you're in. This change keeps group headers pinned at the top of the list as you scroll, so context doesn't disappear as soon as you move past the first few tasks.

• Less disorientation in long lists — when tasks are grouped by date, status, project, or another property, the current group label stays visible instead of scrolling away
• Easier to work inside a group — you can review, reorder, or complete tasks in a section without scrolling back up to remember where you are
• Clearer nested groups — subgroup headers stay visible just below their parent group, so multi-level grouping remains readable while scrolling
• Consistent with other TaskNotes views — similar to sticky headers already used elsewhere in the app, so the behavior feels familiar rather than surprising

Test plan

• Open a grouped task list with enough tasks to scroll
• Scroll through a section and confirm the group name stays visible at the top
• Repeat with subgroups enabled and confirm both parent and subgroup context remain clear
• Collapse and expand a group while scrolled, and confirm the header still behaves naturally
• Check an embedded task list in a note or sidebar to make sure scrolling still feels smooth