docs: clarify Mini App vs Standard Web App migration paths

[Kubudak90]

Summary

This PR addresses the confusion reported in #1247 about when Mini App import + Farcaster manifest are still required vs when the new Standard Web App flow should be used.

Changes

migrate-to-standard-web-app.mdx

• Added a "Which migration path should I use?" decision tree section at the top
• Clear table showing when Farcaster manifest is required vs optional
• Explicit guidance that as of April 9, 2026, most apps should use the Standard Web App flow
• Direct link to the legacy flow for apps that specifically need Farcaster integration

migrate-existing-apps.mdx

• Added prominent deprecation warning at the top
• Clarified this is the legacy flow for Farcaster-specific features only
• Added reference to the new Standard Web App flow as the recommended alternative
• Added "Alternative: Standard Web App Flow" section at the bottom

Why this helps

The current docs have two different pages with conflicting information:

• migrate-to-standard-web-app.mdx says "no Farcaster manifest required"
• migrate-existing-apps.mdx still guides through manifest/import flow

This leaves developers unsure which path to follow. The changes add:

1. A decision tree to help developers choose the right path
2. Clear deprecation notice on the legacy page
3. Cross-links between both pages for easy navigation

Fixes #1247

[cb-heimdall]

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