feat(auth): make reauthentication a navigation destination

[demolaf]

Reauthentication was a host-owned ModalBottomSheet containing its own NavDisplay and back stack. It is now a single destination on the main back stack — AuthRoute.Reauth(requestId, userUid, step) — rendered as a sheet by an OverlayScene scene strategy. One stack means one onBack, one place that can pop, and no way for two stacks to disagree about a back press.

ReauthPresentationState and its saver are gone: the marker rides on the key, so rememberNavBackStack persists it. ReauthSheetContent and CustomReauthContent are gone too — all three were internal. ReauthContentState is unchanged.

• ReauthDestinations.kt / ReauthSceneStrategy.kt: the reauth entry and the sheet/bare presentations, honouring configuration.transitions between steps and hiding the sheet via OverlayScene.onRemove.
• EmailAuthDestinations.kt: the email step body is now a shared EmailAuthStep, so the reachability guard applies to reauth too — a restored stack holding an unoffered step no longer renders a sign-up form mid-reauthentication.
• The MFA challenge entry is pushed and popped from one place, keyed on the state being RequiresMfa.

The reauth surface now stays mounted through RetryingOperation and reports it as loading, rather than disappearing and uncovering the flow behind it.

Added tests for the shared reachability guard, the transition between reauth steps, the MFA challenge pop, and the retry behaviour.

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Maintainer note: Fixes internal CPRN-403

[gemini-code-assist]

Code Review

This pull request refactors the reauthentication flow in FirebaseUI Auth by integrating it directly into the host's back stack as a wrapped destination (AuthRoute.Reauth) and managing its presentation via a custom ReauthSceneStrategy. This replaces the previous separate modal sheet and custom content implementations, which have been removed along with their state savers. Additionally, parallel sync is enabled in gradle.properties. Feedback on these changes suggests using rememberUpdatedState for more idiomatic state tracking in FirebaseAuthScreen and adding a safety check in reauthDestinations to ensure the active reauthentication request ID matches the back stack key before rendering.