Brand the auth screens per tenant

A club member signing in at ajax-united.rosterchief.app now sees their club's
logo, name and colours; the base domain keeps the RosterChief skin for the
control panel and Django admin.

The mechanism is `{% extends base_template %}` -- Django lets the parent be a
context variable, so the `branding` context processor picks the skin from
request.club and *every* auth screen allauth ships (login, password reset, MFA,
passkeys, and whatever it adds next) follows the tenant without a single one of
them knowing that clubs exist.

Templates split three ways: _base.html is the skeleton with no branding, and
_platform_base.html / _club_base.html dress it. The control panel extends the
platform base *explicitly* rather than through the variable, so a bug in
branding resolution can never dress the panel up as a club.

Club gains an optional logo and primary_color. Notes on both:

- No logo falls back to the club's initials, never the RosterChief mark, which
  would pass our branding off as theirs.
- Club colours land in an inline :root. daisyUI declares its theme variables
  inside `@layer base`, and unlayered styles beat every layered rule regardless
  of specificity, so this needs no !important. --color-primary-content is derived
  from WCAG relative luminance, so a club that picks pale yellow gets black text
  instead of invisible white.
- primary_color is a text input, not <input type="color">: a colour picker cannot
  express "no colour", so every club that never touched it would submit #000000
  and silently get a black theme.

"/" now resolves per tenant (club home, or hand off to the control panel), which
is why LOGIN_REDIRECT_URL can stay "/" and allauth needs no redirect adapter.

Also folds in the theme toggle gaining a third "auto" state and the logo
switching from `content:` to background-image (content-replacement on a real
element is not supported in Firefox), both of which lived in the base template
this commit replaces.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

rosterchief/urls.py

@@ -8,16 +8,24 @@ enrolled.
 """
 
 from django.conf import settings
+from django.conf.urls.static import static
 from django.contrib import admin
 from django.urls import include, path
 from django.views.generic import RedirectView
 
+from club.views import root
+
 urlpatterns = [
     path("admin/login/", RedirectView.as_view(pattern_name="account_login", query_string=True), name="admin_login_redirect"),
     path("admin/", admin.site.urls),
     path("accounts/", include("allauth.urls")),
     path("controlpanel/", include("controlpanel.urls")),
+    # "/" resolves per tenant: a club subdomain lands on the club, the base domain
+    # hands off to the control panel. This is why LOGIN_REDIRECT_URL can stay "/".
+    path("", root, name="root"),
 ]
 
 if settings.DEBUG:
     urlpatterns += [path("__reload__/", include("django_browser_reload.urls"))]
+    # Club logos are uploads: runserver has to serve MEDIA_ROOT itself.
+    urlpatterns += static(settings.MEDIA_URL, document_root=settings.MEDIA_ROOT)