[demolaf]

Points the auth README's Theming section at the FirebaseAuthScreen slot APIs, since that section covered restyling but never said whole screens can be replaced.

Changes

• New ### Beyond Theming at the end of Theming & Customization, linking Custom UI with Slots and the fullcustomization demo, which combines a theme with every slot. Added to the TOC.
• Drops "(data class)" from the .copy() bullet: AuthUITheme is a plain class with an explicit copy.

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Maintainer note: Fixes internal CPRN-605

[gemini-code-assist]

Code Review

This pull request updates the auth/README.md file by adding a new 'Beyond Theming' section to the table of contents and the main text. This section explains how to use FirebaseAuthScreen slot parameters to completely replace screen layouts while keeping the authentication logic, referencing a demo app sample. Additionally, a minor correction was made to remove the '(data class)' label from the .copy() theme customization description. There are no review comments, and I have no feedback to provide.