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

## 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