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/coach_mixins.py

@@ -0,0 +1,71 @@
+"""Shared scaffolding for every Coach-mode screen (C1-C6) -- the dark-chrome
+mirror of mobile/mixins.py's PersonScopeMixin. Scoped by *team*, not by
+managed people: a coach acts on one team at a time (the header's team-picker
+pill), switching which team is "active" rather than aggregating across
+several people the way Member mode's person switcher does.
+"""
+
+from club.services.access import current_season, teams_managed_by, teams_staffed_by
+from members.models import Member
+from members.views import ClubScopedPublicMixin
+
+#: Session key remembering which team was last active, so navigating between
+#: coach screens (or leaving and coming back) doesn't reset the picker to
+#: whichever team happens to sort first.
+ACTIVE_TEAM_SESSION_KEY = "coach_active_team_id"
+
+
+class CoachScopeMixin(ClubScopedPublicMixin):
+    """Resolves the signed-in account's Coach-mode standing: which teams
+    they're on the staff of at all (``staffed_teams`` -- visibility, any
+    position, see club.services.access.teams_staffed_by), which of those
+    they actually manage (``managed_teams`` -- management position only,
+    current season, teams_managed_by), and which one is currently "active"
+    (the team-picker pill on C1's header).
+
+    Every Coach view still needs its own ``LoginRequiredMixin`` (kept
+    separate, same as PersonScopeMixin, so a view composes whichever other
+    mixins it needs on top). Nothing here 404s when ``staffed_teams`` is
+    empty -- each screen renders its own "not staffing a team yet" empty
+    state instead, same judgment call as HomeView's "No one to show yet".
+
+    Actions gated on ``can_manage_active_team`` are hidden in templates, not
+    disabled -- a coach on staff but without a management position (e.g. a
+    physio) can see Coach mode but shouldn't see edit affordances they don't
+    have the authority to use.
+    """
+
+    def dispatch(self, request, *args, **kwargs):
+        self.me = Member.objects.filter(user=request.user).first() if request.user.is_authenticated else None
+        self.staffed_teams = list(teams_staffed_by(request.user, request.club)) if request.user.is_authenticated else []
+        self.managed_teams = list(teams_managed_by(request.user, request.club)) if request.user.is_authenticated else []
+        self.active_team = self._resolve_active_team(request)
+        self.can_manage_active_team = self.active_team is not None and self.active_team in self.managed_teams
+        return super().dispatch(request, *args, **kwargs)
+
+    def _resolve_active_team(self, request):
+        requested_id = request.GET.get("team")
+        for team in self.staffed_teams:
+            if requested_id and str(team.pk) == requested_id:
+                request.session[ACTIVE_TEAM_SESSION_KEY] = str(team.pk)
+                return team
+
+        stored_id = request.session.get(ACTIVE_TEAM_SESSION_KEY)
+        for team in self.staffed_teams:
+            if stored_id and str(team.pk) == stored_id:
+                return team
+
+        return self.staffed_teams[0] if self.staffed_teams else None
+
+    def get_context_data(self, **kwargs):
+        kwargs.setdefault("active_tab", getattr(self, "active_tab", ""))
+        kwargs.setdefault("screen_title", getattr(self, "screen_title", ""))
+
+        return super().get_context_data(
+            me=self.me,
+            staffed_teams=self.staffed_teams,
+            active_team=self.active_team,
+            can_manage_active_team=self.can_manage_active_team,
+            season=current_season(self.request.club),
+            **kwargs,
+        )