fix(applier): preserve renamed/recolored groups on re-group

Previously applyPlans called tabGroups.update() unconditionally for every
plan, clobbering manual renames, recolors, and collapse state on each
re-group even with 'ungroup before grouping' unchecked.

Now reuse happens by membership (the group the plan's tabs already live in),
and title/color/collapsed are set only when creating a brand-new group. The
controller passes candidates through so membership reuse works in production.

Adds tests: re-grouping preserves a renamed group; reuse makes no update
call; new groups still get their appearance set.