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>

controlpanel/templates/controlpanel/base.html

@@ -1,4 +1,6 @@
-{% extends "base.html" %}
+{# Hardcoded, not `base_template`: the panel is platform-only, and a branding bug
+   must never be able to dress it up as a club. #}
+{% extends "_platform_base.html" %}
 {% load lucide %}
 
 {% block title %}

controlpanel/templates/controlpanel/club_form.html

@@ -6,7 +6,7 @@
 {% block panel %}
     <div class="card max-w-xl bg-base-100 shadow">
         <div class="card-body">
-            <form method="post">
+            <form method="post" enctype="multipart/form-data">
                 {% csrf_token %}
                 {% for error in form.non_field_errors %}
                     <div class="alert alert-error my-2">