Highlight Squad on the Add player/staff screens, add staff removal

- CoachAddPlayerView/CoachAddStaffView now set active_tab = "coach_squad"
  instead of "coach_today" -- both are reached from Squad's own "Add"
  buttons, so that's the tab that should stay highlighted while there.
- Squad screen: each staff row (other than your own) gets a remove button,
  mirroring the roster's own. Self-removal stays a desktop-only action
  (management.views.TeamStaffRemoveView) -- doing it from here would strand
  a coach off a team they're actively viewing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

mobile/urls.py

@@ -35,6 +35,7 @@ urlpatterns = [
     path("coach/news/new/", coach_views.CoachCreateNewsView.as_view(), name="coach_create_news"),
     path("coach/roster/add/", coach_views.CoachAddPlayerView.as_view(), name="coach_add_player"),
     path("coach/staff/add/", coach_views.CoachAddStaffView.as_view(), name="coach_add_staff"),
+    path("coach/staff/<uuid:assignment_pk>/remove/", coach_views.CoachStaffRemoveView.as_view(), name="coach_staff_remove"),
     path("coach/lineup/<uuid:event_id>/", coach_views.CoachLineupView.as_view(), name="coach_lineup"),
     path("coach/lineup/<uuid:event_id>/publish/", coach_views.CoachLineupPublishView.as_view(), name="coach_lineup_publish"),
 ]