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>

club/migrations/0012_club_logo_club_primary_color.py

@@ -0,0 +1,25 @@
+# Generated by Django 6.0.6 on 2026-07-13 17:33
+
+import club.models
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('club', '0011_alter_club_slug'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='club',
+            name='logo',
+            field=models.ImageField(blank=True, help_text="Shown on the club's own pages. Without one, the club's initials are used.", upload_to=club.models.club_logo_path, verbose_name='logo'),
+        ),
+        migrations.AddField(
+            model_name='club',
+            name='primary_color',
+            field=models.CharField(blank=True, help_text="Hex colour for buttons and links on the club's pages, e.g. #1e40af.", max_length=7, validators=[django.core.validators.RegexValidator('^#[0-9a-fA-F]{6}$', 'Enter a colour as a hex value, e.g. #1e40af.')], verbose_name='primary colour'),
+        ),
+    ]