docs(auth): update Screen Transitions samples to the Navigation 3 AuthUITransitions

[demolaf]

The Screen Transitions samples still use the Navigation 2 constructor — enterTransition / exitTransition / popEnterTransition / popExitTransition — which no longer exists, so none of the four compile. AuthUITransitions now takes one ContentTransform per direction: transitionSpec, popTransitionSpec and predictivePopTransitionSpec.

Rewrites the four samples against the new constructor, pairing each enter/exit with togetherWith, and adds a per-destination sample using the public Scene<NavKey>.authRoute(). The closing note now says each spec falls back to the default 700ms cross-fade independently — predictivePopTransitionSpec does not fall back to popTransitionSpec, and it fires when the gesture starts rather than when a back completes.

Compiled all five samples verbatim against the module to check they build.

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Maintainer note: Fixes internal CPRN-404

[gemini-code-assist]

Code Review

This pull request updates the auth/README.md documentation to reflect changes in screen transitions, moving from individual enter/exit transitions to unified transition specs using togetherWith. It also adds documentation for per-destination animations and updates notes on default transitions. Feedback is provided regarding an inconsistency in the vertical slide transition example, where the exit animation direction contradicts the accompanying comment.