Scaffold the mobile member app: PWA shell, push, and app-shell tokens

New `mobile` Django app mounted at /app/ -- the installed PWA for Member
mode (M1-M7, see design_handoff_rosterchief_platform/README.md). Coach
mode (C1-C6) is a later phase and has no routes yet.

Foundation pieces:
- assets/mobile.css: Tailwind v4 theme reusing management.css's design
  tokens (same per-club --tenant-* theming pattern), plus the ice/coach
  accent and mobile's 14px card radius.
- PushSubscription model + pywebpush-based sender (mobile/services/push.py),
  wired to notifications.Notification via a post_save signal so the
  existing notification system gains a push channel without knowing about
  PWAs itself.
- Per-club manifest.webmanifest + service worker (served at /app/sw.js)
  + a server-rendered fallback home-screen icon (club initials on
  secondary_color) for clubs without an uploaded logo -- confirmed with
  the user as the fallback, never a generic RosterChief mark.
- App shell (base.html): navy header, person switcher (every child a
  signed-in parent manages, plus "Me"), bottom tab bar, safe-area insets.
  Vendored htmx + Alpine for the screens built on top of it.
- Placeholder views/routes for all seven M1-M7 screens so the shell is
  fully wired end-to-end before each screen is built out individually.

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

mobile/models.py

@@ -0,0 +1,38 @@
+from django.db import models
+from django.utils.translation import gettext_lazy as _
+
+from members.models import Member
+from rosterchief.base import ClubScopedModel, validate_club_scope
+
+
+class PushSubscription(ClubScopedModel):
+    """One browser/device's Web Push registration for one member -- created by
+    the subscribe flow in mobile/static/mobile/app.js, sent to whenever
+    mobile.signals pushes a new notifications.Notification for that member.
+
+    `endpoint` (the URL the browser's own push service gave it) is globally
+    unique by construction -- a browser only ever has one push registration
+    per origin -- so it doubles as the natural "already subscribed on this
+    device" key: re-subscribing replaces the row rather than creating a
+    second one for the same browser (see mobile.views.PushSubscribeView).
+    """
+
+    member = models.ForeignKey(Member, on_delete=models.CASCADE, related_name="push_subscriptions", verbose_name=_("member"))
+    endpoint = models.URLField(_("endpoint"), max_length=500, unique=True)
+    p256dh = models.CharField(_("p256dh key"), max_length=255)
+    auth = models.CharField(_("auth key"), max_length=255)
+    user_agent = models.CharField(_("user agent"), max_length=255, blank=True)
+
+    class Meta:
+        verbose_name = _("push subscription")
+        verbose_name_plural = _("push subscriptions")
+        ordering = ["-created"]
+
+    def __str__(self):
+        return f"{self.member} — {self.user_agent or self.endpoint[:40]}"
+
+    def clean(self):
+        validate_club_scope(self, self.club_id, member_fields=("member",))
+
+    def as_subscription_info(self) -> dict:
+        return {"endpoint": self.endpoint, "keys": {"p256dh": self.p256dh, "auth": self.auth}}