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

pyproject.toml

@@ -23,6 +23,7 @@ dependencies = [
     "psycopg[binary]>=3.3.4",
     "python-dateutil>=2.9.0.post0",
     "python-decouple>=3.8",
+    "pywebpush>=2.0.3",
     "requests>=2.34.2",
     "weasyprint>=69.0",
     "whitenoise>=6.12.0",
@@ -64,7 +65,7 @@ ignore = [
 
 [tool.ruff.lint.isort]
 known-first-party = [
-    "api", "billing", "authentication", "club", "members", "teams", "events", "formbuilder", "shop", "controlpanel", "management", "news", "notifications", "pages", "home", "search", "rosterchief"]
+    "api", "billing", "authentication", "club", "members", "teams", "events", "formbuilder", "shop", "controlpanel", "management", "mobile", "news", "notifications", "pages", "home", "search", "rosterchief"]
 
 [tool.uv.sources]
 django-lucide = { git = "https://github.com/bsiebens/lucide" }