Redesign the Coach mode shell: ice-blue header, flat sheet, Squad/Schedule tabs

Reworks the coach shell per feedback that the previous dark-ink header
didn't read as visually distinct from Member mode's own navy header, and
that the rounded "sheet overlaps header" treatment wasn't landing:

- .coach-header is now solid ice-blue (--color-ice, never club-themed --
  Coach mode's own signature colour) with --color-ice-ink foreground text,
  instead of dark ink with white text. Every header_extra block across the
  coach templates (attendance, lineup) is recoloured to match.
- .coach-sheet drops the 20px rounded-top/negative-margin overlap -- flush
  edge-to-edge below the header now, same join the member shell already uses.
- The header's "Head coach" label was hardcoded regardless of the account's
  actual role -- CoachScopeMixin now resolves active_team_role from the
  person's own current-season StaffAssignment.position on the active team,
  so a team manager or physio sees their real title, not someone else's.

Tab bar is now Today / Squad / [+] / Schedule -- no Me tab (the account's
own settings already live in Member mode via the role switcher, no need
for a second one). Squad (CoachSquadView) is a new roster+staff screen for
the active team; Schedule (CoachScheduleView) is a new full upcoming-events
list for the team, each row routed straight to the coach-relevant action
(Attendance for a practice, Line-up for a game) rather than the Member-
shell RSVP page. The "+" is a raised ice-blue circle opening a small popup
with New event/New post/Add player -- three genuinely different actions,
so the button opens a menu rather than committing to one destination.
Today's own inline New event/New post/Add player buttons are gone now that
the tab bar covers the same ground.

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

static/css/app.css

@@ -2696,6 +2696,9 @@
   .inset-0 {
     inset: 0;
   }
+  .inset-x-3 {
+    inset-inline: calc(var(--spacing) * 3);
+  }
   .dropdown-right {
     @layer daisyui.l1.l2 {
       --anchor-h: right;
@@ -4618,6 +4621,12 @@
       background-color: color-mix(in oklab, var(--color-white) 15%, transparent);
     }
   }
+  .bg-white\/40 {
+    background-color: color-mix(in srgb, #fff 40%, transparent);
+    @supports (color: color-mix(in lab, red, red)) {
+      background-color: color-mix(in oklab, var(--color-white) 40%, transparent);
+    }
+  }
   .btn-link {
     @layer daisyui {
       text-decoration-line: underline;