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

assets/mobile.css

@@ -445,4 +445,49 @@
         color: #fff;
         border-radius: var(--radius-box);
     }
+
+    /* --- Coach mode shell (C1-C6) ---------------------------------------------------
+       Dark-chrome mirror of the member .app-header/.tab-bar pair above. .coach-sheet is
+       the mode's signature: a light body that overlaps the ink header by 20px via a
+       matching negative margin, so the header appears to sit "behind" a rounded sheet
+       rather than the two stacking edge-to-edge like the member shell's navy header does. */
+
+    .coach-header {
+        padding: max(env(safe-area-inset-top), 14px) 16px 14px;
+        background: var(--color-ink);
+        color: #fff;
+        flex-shrink: 0;
+    }
+
+    .coach-sheet {
+        background: var(--color-paper);
+        border-radius: 20px 20px 0 0;
+        margin-top: -20px;
+        position: relative;
+        flex: 1;
+        overflow-y: auto;
+    }
+
+    .coach-tab-bar {
+        flex-shrink: 0;
+        background: var(--color-ink);
+        border-top: 1px solid var(--color-hairline);
+        padding: 8px 8px max(env(safe-area-inset-bottom), 26px);
+        display: flex;
+    }
+
+    .coach-tab-bar-item {
+        flex: 1;
+        height: 48px;
+        display: flex;
+        flex-direction: column;
+        align-items: center;
+        justify-content: center;
+        gap: 3px;
+        color: var(--color-on-dark);
+    }
+
+    .coach-tab-bar-item-active {
+        color: var(--color-ice);
+    }
 }