feat: implement legacyFetchSignInWithEmail behaviour for Angular and React

[russellwheatley]

• Adds a new legacyFetchSignInWithEmail behavior in packages/core to handle auth/account-exists-with-different-credential and related email/password recovery cases.
• Fetches sign-in methods for the conflicting email and stores recovery state on the FirebaseUI instance.
• Persists pending OAuth credentials so they can be linked after the user signs in with the existing provider.
• Adds legacySignInRecovery state plus setter/clear helpers to the core config/store.
• Updates core auth/error handling so email context is available when building recovery state.
• Adds default legacy recovery UI for React.
• Adds default legacy recovery UI for Angular.
• Adds React APIs to consume or suppress the built-in recovery flow.
• Adds Angular APIs to consume or suppress the built-in recovery flow.
• Updates docs/examples/translations for the new recovery flow.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a robust mechanism for handling authentication errors where a user attempts to sign in with a different provider than previously used for the same email. It provides a new behavior that fetches and presents alternative sign-in methods, along with corresponding UI components for React and Angular, to guide users through the recovery process. This significantly improves the user experience by preventing dead ends during sign-in and offering clear paths to resolve credential conflicts.

Highlights

• New Legacy Sign-In Recovery Behavior: Introduced a new legacyFetchSignInWithEmail behavior in @firebase-oss/ui-core to augment OAuth auth/account-exists-with-different-credential flows. This behavior fetches and stores available sign-in methods for a user's email, enabling recovery from mismatched credentials.
• Cross-Platform UI Components for Recovery: Added dedicated UI components (LegacySignInRecoveryComponent for Angular and LegacySignInRecovery for React) to automatically render the recovery state as a modal on SignInAuthScreen and OAuthScreen. These components guide users to sign in with their previously used method.
• Granular UI Control: Provided showLegacySignInRecovery props for SignInAuthScreen and OAuthScreen in both React and Angular, allowing developers to hide the built-in recovery UI and implement custom recovery flows using new hooks/injectors (useLegacySignInRecovery for React, injectLegacySignInRecovery/injectClearLegacySignInRecovery for Angular).
• Enhanced Error Handling: Modified the core error handling logic to clear legacy sign-in recovery state and attach email information to errors, improving the context provided during authentication failures.
• Documentation and Examples: Updated the README.md with comprehensive documentation for the new legacyFetchSignInWithEmail behavior and its associated UI components and hooks. A new demo screen was added to the React example application to showcase the recovery flow.

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[russellwheatley]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a new legacyFetchSignInWithEmail behavior to handle OAuth account mismatch flows, allowing users to recover their accounts by suggesting previously used sign-in methods. The changes include core logic for error handling and state management, new UI components for React and Angular to display recovery options, and updated translations. My feedback suggests a minor improvement to the attachEmailToError function to ensure safer cloning of the customData object.