[Docs] Post-OP-Stack migration: obsolete references and missing post-deadline guidance

[lau90eth]

Summary

Following Base's exit from Optimism's OP Stack (Feb 2026) and the April 9 migration deadline for Farcaster mini-apps, I found several docs inconsistencies.

Issues Found

1. Obsolete OP Stack reference in fee docs

   • Page: /base-chain/network-information/network-fees
   • Text links to "op-stack developer documentation"
   • Base now runs the unified base/base stack

2. Missing post-deadline guidance in migration guide

   • Page: /apps/quickstart/migrate-to-standard-web-app
   • Deadline was April 9, 2026
   • No section explaining what happens to unmigrated apps (grace period? hard stop? error messages?)

3. README base/node still claims "built on Optimism's OP Stack"

   • Should reflect the unified stack architecture

4. README base/contracts says "utilizes Optimism's bedrock contracts"

   • Needs clarification post-migration

Suggested Fixes

• Update fee docs to reference Base's own fee mechanics or clarify OP Stack heritage
• Add "Post-April 9" FAQ to migration guide
• Update base/node and base/contracts READMEs to reflect current architecture