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

static/css/app.css

@@ -4067,6 +4067,9 @@
   .w-20 {
     width: calc(var(--spacing) * 20);
   }
+  .w-23 {
+    width: calc(var(--spacing) * 23);
+  }
   .w-24 {
     width: calc(var(--spacing) * 24);
   }
@@ -4847,6 +4850,9 @@
   .pb-3 {
     padding-bottom: calc(var(--spacing) * 3);
   }
+  .pb-4 {
+    padding-bottom: calc(var(--spacing) * 4);
+  }
   .pb-5 {
     padding-bottom: calc(var(--spacing) * 5);
   }