feat: add custom branding for signing pages (#2785)

Platform-plan organisations and teams can now customise non-embed
signing pages with six brand colour tokens, a border-radius, and
a free-text custom CSS block (up to 256 KB).

- Stored on OrganisationGlobalSettings / TeamGlobalSettings;
  teams inherit from the org via brandingEnabled === null.
- CSS is sanitised on save (PostCSS) so we can inline it at SSR
  with no per-render parsing.
- Rendered via a nonce'd <style> scoped under .documenso-branded,
  using native CSS nesting so user selectors don't need scoping.
- Gated on the existing embedSigningWhiteLabel claim (or
  self-hosted) — reuses the embed white-label decision.

packages/auth/server/lib/utils/handle-oauth-callback-url.ts

@@ -1,8 +1,5 @@
 import { NEXT_PUBLIC_WEBAPP_URL } from '@documenso/lib/constants/app';
-import {
-  isEmailDomainAllowedForSignup,
-  isSignupEnabledForProvider,
-} from '@documenso/lib/constants/auth';
+import { isEmailDomainAllowedForSignup, isSignupEnabledForProvider } from '@documenso/lib/constants/auth';
 import { AppError, AppErrorCode } from '@documenso/lib/errors/app-error';
 import { onCreateUserHook } from '@documenso/lib/server-only/user/create-user';
 import { deletedServiceAccountEmail } from '@documenso/lib/server-only/user/service-accounts/deleted-account';