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>

static/src/app.css

@@ -166,10 +166,29 @@
     --font-tourney: "Tourney", ui-sans-serif, system-ui, sans-serif;
 }
 
+/* The logo is a background image, not `content:` -- content-replacement on a real
+   element (rather than ::before/::after) isn't supported in Firefox.
+
+   Default = dark-ink logo, for a light background. The media query covers "auto",
+   where the toggle deliberately sets no data-theme at all; the attribute selectors
+   are more specific, so an explicit choice always beats the OS. */
+.logo {
+    background-image: var(--logo-dark);
+    background-position: center;
+    background-repeat: no-repeat;
+    background-size: contain;
+}
+
+@media (prefers-color-scheme: dark) {
+    .logo {
+        background-image: var(--logo-light);
+    }
+}
+
 [data-theme="light"] .logo {
-    content: var(--logo-dark);
+    background-image: var(--logo-dark);
 }
 
 [data-theme="dark"] .logo {
-    content: var(--logo-light);
+    background-image: var(--logo-light);
 }