fix(auth): pre-fill email when "Continue as" button is tapped (#2423)

[just1and0]

Summary

• The "Continue as…" button in AuthMethodPicker displayed the saved email/identifier but discarded it on click — the user landed on a blank email form identical to tapping the regular provider button.
• Added an onContinueAsSelected callback to AuthMethodPicker that carries the saved identifier through to EmailAuthScreen, which now pre-fills the email field with the saved address.
• Regular provider button taps clear the pre-fill so the two paths remain distinct.

Fixes #2423

My.Movie.2.mp4

Test plan

• Sign in with email, sign out, return to the auth method picker
• Tap the "Continue as " button — verify the email field is pre-filled with the saved address
• Tap the regular "Sign in with email" button below the divider — verify the email field is empty
• Verify federated providers (Google, Facebook, etc.) are unaffected by the change
• Verify custom layouts via customLayout still work (they don't receive onContinueAsSelected)

[gemini-code-assist]

Code Review

This pull request introduces the ability to pre-fill the email address on the email authentication screen when a user selects the "Continue as..." option from the method picker. This is achieved by passing the saved identifier from AuthMethodPicker via a new onContinueAsSelected callback, storing it in a prefillEmail state in FirebaseAuthScreen, and passing it to EmailAuthScreen to initialize the email input field. Feedback on the changes highlights a potential issue where a non-email identifier (like a phone number) could incorrectly pre-fill the email field if the last used provider was not Email. A code suggestion is provided to only set prefillEmail when the provider is AuthProvider.Email.