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/templates/mobile/coach/squad.html

@@ -60,6 +60,15 @@
                             <div class="text-[15px] font-semibold text-ink">{{ assignment.member.get_full_name }}</div>
                             <div class="text-xs text-muted">{{ assignment.position }}</div>
                         </div>
+                        {% if can_manage_active_team and assignment.member_id != me.pk %}
+                            {% trans "Remove this staff member?" as remove_staff_confirm_text %}
+                            <form method="post" action="{% url "mobile:coach_staff_remove" assignment.pk %}" hx-boost="false" onsubmit="return confirm('{{ remove_staff_confirm_text|escapejs }}')">
+                                {% csrf_token %}
+                                <button class="flex h-9 w-9 shrink-0 items-center justify-center rounded-lg text-club" type="submit" aria-label="{% trans "Remove" %}">
+                                    {% lucide "trash-2" size=16 %}
+                                </button>
+                            </form>
+                        {% endif %}
                     </div>
                 {% empty %}
                     <div class="p-6 text-center text-sm text-muted">{% trans "No staff assigned for this season yet." %}</div>