Tasks TUI:
- Add context support to TaskBackend interface (get_context, set_context,
get_contexts methods)
- Implement context methods in DstaskClient
- Add Context section to FilterSidebar (above projects/tags)
- Context changes persist via backend CLI
Calendar TUI:
- Remove duplicate header from InvitesPanel (use border_title instead)
- Fix border_title color to use $primary
- Fix WeekGrid to always scroll to work day start (7am) on mount
