feat(auth): implement legacyFetchSignInWithEmail configuration option

[russellwheatley]

• Adds legacyFetchSignInWithEmail to AuthUIConfiguration, defaulting to false.
• Opts into legacy fetchSignInMethodsForEmail() recovery only for projects that have email enumeration protection disabled.
• Improves the email/password sign-in flow when a user tries the wrong auth method for an existing account.
• Detects when an email/password attempt should use a different configured provider instead of password sign-in.
• Surfaces provider-aware recovery actions from the email error dialog, including direct continuation to Google/Facebook/etc. or email-link sign-in when applicable.
• Preserves existing behavior by default when the new flag is not enabled.
• Adds focused unit test coverage for configuration defaults and legacy provider-recovery behavior.
• Fixes top-level error dialog recovery handling so custom recovery actions do not override normal retry behavior.
• helper script to run demo app from command line.

Screen recording shows my attempt at trying to sign-in with Github when I have an account with email provider only:

Screen_recording_20260420_132518.webm

[gemini-code-assist]

Code Review

This pull request implements a legacy recovery path for email sign-in, intended for use when email enumeration protection is disabled. It introduces the DifferentSignInMethodRequiredException and a legacyFetchSignInWithEmail configuration flag. The core logic uses the legacy fetchSignInMethodsForEmail API to suggest alternative providers when a user attempts to sign in with an incorrect method. The UI layer, including ErrorRecoveryDialog and various screens, has been updated to handle this recovery flow. Review feedback highlights a missing recovery case in the top-level FirebaseAuthScreen, a redundant check in the recovery logic, and the need to support generic OAuth providers in the provider redirection callback.