Fix Webflow app review findings

[simple-analytics-ai]

Depends on #3. Merge #3 first; this PR is intentionally based on its chore/import-webflow-app branch.

Summary

• include the configured OAuth callback URI in the access-token exchange
• replace local-only disconnect with authenticated script cleanup, encrypted-token deletion, and final-grant revocation
• preserve saved per-site settings while the Designer site ID loads
• remove unsupported duplicate-script detection and show clear manual-migration guidance instead
• correct the setup documentation for this repository, Node 24, and Redis

Validation

• npm test --prefix data-client (9 passed)
• npm run lint --prefix data-client
• npm run lint --prefix designer-extension
• npm run build:prod --prefix designer-extension
• npm run build --prefix data-client
• Redis lifecycle checks for shared grants, legacy records, and token rotation
• npm audit: Designer Extension clean; backend has five low-severity transitive findings through webflow-api 3.3.4, which remains the latest release and has no audit fix available

Deployment dependency

PR #3 remains dependent on simpleanalytics/infrastructure#275 for production secrets, Redis, ingress, and Caddy configuration.