fix: allow invited users to sign up when registration is disabled (#418)

* fix: allow invited users to sign up when registration is disabled

Move sign-up restriction logic from better-auth's disableSignUp config
to the existing user.create.before database hook, which already checks
for pending invitations. The frontend signup and login pages now detect
invite flows (?next=/invite/...) and bypass the disabled UI accordingly.

Closes #411

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* test: add regression tests for sign-up hook invite bypass

Verify that the user.create.before database hook correctly:
- allows sign-up when registration is not disabled
- blocks sign-up when disabled and no invitation exists
- allows sign-up when disabled but a pending invitation exists
- respects BETTER_AUTH_ALLOWED_DOMAINS in combination with invites

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* test: add OIDC/social sign-up path coverage for invite bypass

Address review suggestion: add explicit tests verifying the
user.create.before hook handles OIDC/social sign-ups the same way as
email/password — invited users are allowed, uninvited users are blocked.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/views/auth/login/index.tsx

@@ -58,7 +58,7 @@ export default function LoginPage() {
                 </div>
               </div>
             )}
-            {!isSignUpDisabled && (
+            {(!isSignUpDisabled || redirect?.startsWith("/invite/")) && (
               <p className="mt-4 text-sm text-light-1000 dark:text-dark-1000">
                 <Trans>
                   Don't have an account?{" "}

apps/web/src/views/auth/signup/index.tsx

@@ -28,7 +28,9 @@ export default function SignUpPage() {
     setMagicLinkRecipient(recipient);
   };
 
-  if (isSignUpDisabled) {
+  const isInviteFlow = redirect?.startsWith("/invite/");
+
+  if (isSignUpDisabled && !isInviteFlow) {
     return (
       <>
         <PageHead title={t`Sign up | kan.bn`} />