feat(groups): Phase 1 group mode UX — enhanced HUD, lifecycle fix, groups/tags distinction
Three changes for 'Feel the group' UX:
1. Enhanced HUD for group mode: Shows colored accent stripe and prominent
group name banner when mode is 'group'. Uses the group's color from
metadata for visual theming.
2. Fix group mode lifecycle: Closing the groups panel no longer resets
group mode on all member windows. Group mode persists until the user
explicitly navigates back to the groups list or changes mode.
3. Groups vs Tags distinction via isGroup metadata flag: The groups UI
now filters to show only tags promoted to groups (isGroup: true in
tag metadata). Unpromoted tags are accessible via an 'All Tags'
toggle with promote buttons. Creating a group via the UI or saving
to a group auto-sets isGroup: true.