Add registration/access control to PasswordProvider and CodeProvider

[santiagomera]

Summary

Fix #199

This PR adds flexible registration control to the PasswordProvider, enabling scenarios where sign-ups are restricted or completely disabled while supporting admin-created users through external system.

New Configuration Options

allowRegistration - Controls whether new user registration is allowed:

• boolean: true (default) allows user registration, false disables user registration
• function: (email: string) => boolean | Promise<boolean> for custom validation logic (e.g., domain restrictions)

userExists - Callback to check if a user exists in an external system:

• (email: string) => boolean | Promise<boolean>
• Used during password reset flow to validate existing users when registration is disabled
• Only invoked when allowRegistration === false

Implementation Details

Registration Flow Changes

• When allowRegistration: false: Registration routes redirect to login page
• When allowRegistration: function: Function validates each registration attempt on form submission
• UI updates: Registration links conditionally hidden when disabled

Password Reset Flow for Admin-Created Users

First-time log in when registration is disabled is handled with the existing password reset flow (new users need to reset their password before being able to log in).
At the password update step, if the user if not found in Storage and the user exists in external system, it is created in Storage and the regular reset logic continues.

Examples

Admin-Created User First Login

1. Admin creates user "john@company.com" in external system (no OpenAuth storage entry)
2. User visits login page, sees "First time logging in? Reset your password" link
3. User clicks reset link, enters email, receives verification code
4. User enters code, proceeds to password setting screen
5. When user sets password:
   • OpenAuth checks storage → user not found
   • OpenAuth calls userExists("john@company.com") → returns true
   • OpenAuth creates storage entry for user
   • User completes password setup successfully
6. Future logins work normally

Domain-Restricted Registration

PasswordProvider(PasswordUI({
  allowRegistration: (email) => {
    return email.endsWith("@company.com") || email.endsWith("@contractor.com")
  },
  sendCode: sendEmailCode
}))

Disabled Registration with External Users

PasswordProvider(PasswordUI({
  allowRegistration: false,
  userExists: async (email) => {
    return await corporateDirectory.userExists(email)
  },
  sendCode: sendEmailCode
}))

Breaking Changes

There shouldn't be.

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Latest commit: 80169d9

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[santiagomera]

Added access control to the CodeProvider since it's just a similar but simpler case than the PasswordProvider.