[Feature]: Expand custom UI docs and examples

[Bccorb]

Problem statement

Summary

The package architecture now clearly supports custom auth UIs through:

• AuthProvider
• useAuth()
• createSeamlessAuthClient()
• useAuthClient()
• usePasskeySupport()

But the docs still lean more heavily on the built-in routes and basic provider setup than on full custom flow examples.

Goal

Improve docs so the headless-capable SDK path is as easy to adopt as the bundled AuthRoutes path.

Areas to cover

• custom login screen using useAuthClient()
• custom registration flow
• when to call refreshSession()
• passkey support detection
• magic-link / OTP continuation examples
• credential management examples using provider helpers

Proposed solution

Acceptance criteria

• README includes at least one fuller custom login example
• README includes at least one fuller custom registration or post-auth sync example
• refreshSession() usage is clearly documented
• Docs make it obvious when to choose:
  • AuthRoutes
  • useAuth()
  • useAuthClient()
  • createSeamlessAuthClient()

Alternatives considered

No response