docs(auth): correct the auth README against the current API

[demolaf]

The auth README documented an API in several places that never existed. §AuthFlowController and §Low-Level API both showed val state = controller.start() returning an AuthState from a coroutine - the real start() takes an Activity and a request code, returns Unit, and is deprecated. Both now register an ActivityResultLauncher and launch controller.createIntent(this), matching AuthFlowControllerDemoActivity. state.result.user became state.user, since AuthState.Success.result is nullable and user is the field being reached for.

Smaller corrections in the same vein: MfaChallengeScreen was passing a non-existent onChallengeComplete while omitting the required auth and onSuccess; ErrorRecoveryDialog was missing the required stringProvider; FirebaseAuthUI.create was missing the required app; two providers = listOf(...) assignments became the providers { } block, whose setter is private; and context is now set in all 40 authUIConfiguration {} examples - the builder requires it and throws without it, and only 3 set it.

isCredentialLinkingEnabled, passwordResetActionCodeSettings and legacyFetchSignInWithEmail are all read by the library but were documented nowhere, so they are now in the configuration reference. Minimum requirements said SDK 21 and Kotlin 1.9 against minSdk = 23 and Kotlin 2.4, and the version checklist and Compose BOM were stale.

§Migration Guide is replaced by a link to docs/upgrade-to-10.0.md. It restated that file's own key-changes list and checklist, the two had drifted apart, and the OKF documentation policy asks for one owning doc per topic.

§Screen Transitions is #2475 absorbed into this PR rather than rewritten here - its two commits are cherry-picked with authorship intact, so the samples are the ones already compiled verbatim against the module. #2475 is closed in favour of this one. The only addition is the context line, for consistency with the rest of the file.

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Maintainer note: Fixes internal CPRN-404
Maintainer note: Refs internal CPRN-427

[gemini-code-assist]

Code Review

This pull request updates the auth/README.md documentation to reflect new minimum requirements (Android SDK 23+, Kotlin 2.0+), API changes requiring an application context in authUIConfiguration, and updated transition animations using togetherWith. The review feedback correctly identifies several issues in the updated Compose code snippets, such as the unavailability of applicationContext inside standalone Composable functions, the incorrect usage of LocalContext.current inside non-composable lambdas, and the need to clarify that registerForActivityResult must be called during initialization.

[russellwheatley]

LGTM. The fake controller.start() return value is gone, and the migration section now points at the upgrade guide instead of drifting from it.