feat(docs): Add "Transactional Farcaster Frames" Cookbook Entry

[jadonamite]

Overview

This PR adds a comprehensive guide to building Transactional Frames on Base, enabling users to mint NFTs directly from their social feed.

Changes

• New File: apps/base-docs/docs/cookbook/farcaster/transactional-frames.md
• Architecture: Explained the difference between fc:frame:button logic (POST requests) and standard redirects.
• Implementation: Provided full code for a Next.js implementation using OnchainKit and Viem:
  • Metadata: Setting up the tx button action.
  • API Route: Handling the signed POST request and returning encoded calldata.
  • Success State: Updating the Frame UI after the transaction is submitted.

Why

Frames are the primary growth driver on Base right now. This guide fills a gap in the documentation by moving beyond "Read-Only" frames to "Write-Action" frames, which are essential for ecosystem apps.

[cb-heimdall]

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