Add Facebook iOS SDK setup guide

[Ais_C (DeterminedCamel0204)]

Why

The BookCatalog repository has no iOS client, but a companion app may need to integrate Meta authentication or sharing with its API. This guide documents a safe, reproducible starting point without changing the legacy ASP.NET application.

What changed

Added a Facebook iOS SDK setup guide covering Swift Package Manager installation, Meta app and Info.plist configuration, SDK initialization, API integration boundaries, and privacy considerations. It also cautions against trusting client-provided identities without server-side token validation.

Validation

Documentation-only change; no application behavior changed.

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