Add Coach mode: dark-chrome shell, mode switcher, C1 Today, C2 Bench attendance

First slice of the coach-mode build (design_handoff_rosterchief_platform/
README.md's "Coach mode" section, C1-C6). Ships the foundation together with
the two screens the design doc calls out as the reason coaches install
anything at all, rather than landing a Today screen with a dead "Check
attendance" button:

- mobile/coach_mixins.py's CoachScopeMixin -- the dark-mode mirror of
  PersonScopeMixin, scoped by active team (via club.services.access'
  teams_staffed_by/teams_managed_by) instead of managed people.
- A standalone dark ink/ice shell (mobile/templates/mobile/coach/base.html)
  with the mode's signature 20px-radius overlapping sheet, reusing the
  --color-ice/--color-ink tokens that already existed in assets/mobile.css
  but were unconsumed until now.
- The Coach/Member role switcher is re-added to the member shell, gated on
  a real staff assignment (has_coach_access), replacing the "deliberately
  not rendered yet" placeholder.
- C1 Today: stat tiles, a tonight's-session card, a silent-players "needs
  you" row, and an "Also yours" card reusing HomeView's own hero-RSVP
  pattern scoped to the coach's own member record.
- C2 Bench attendance: writes through events.services.attendance.
  record_check_in, which existed for exactly this and had no caller yet.
  Read-only for staff on a team without a management position.

Line-up (C3), create event (C4), post news (C5), and add-to-roster (C6)
follow in later stages -- see the coach-mode plan.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ECGMEwrc2k4D8VQuwjstj9

mobile/urls.py

@@ -1,6 +1,6 @@
 from django.urls import path
 
-from . import views
+from . import coach_views, views
 
 app_name = "mobile"
 
@@ -22,4 +22,7 @@ urlpatterns = [
     path("me/calendar-sync/", views.CalendarFeedSettingsView.as_view(), name="calendar_feed_settings"),
     path("me/<uuid:member_id>/edit/", views.EditProfileView.as_view(), name="edit_profile"),
     path("notifications/", views.NotificationsView.as_view(), name="notifications"),
+    # Coach mode (C1-C6).
+    path("coach/", coach_views.CoachTodayView.as_view(), name="coach_today"),
+    path("coach/attendance/<uuid:event_id>/", coach_views.CoachAttendanceView.as_view(), name="coach_attendance"),
 ]