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>

packages/auth/package.json

@@ -21,6 +21,7 @@
     "dev": "tsc",
     "format": "prettier --check . --ignore-path ../../.gitignore",
     "lint": "eslint",
+    "test": "vitest run",
     "typecheck": "tsc --noEmit --emitDeclarationOnly false"
   },
   "devDependencies": {