Wire Folder Studio Bench preview requests

[cbusillo]

Summary

• wire the Folder Studio Bench composer to the existing preview endpoint
• persist returned preview proposals locally so the transcript and draft table update immediately
• add host/request blocker logic with focused unit coverage

Closes #30

Validation

• bash scripts/pre-commit-check.sh
• Browser QA at http://127.0.0.1:4281/folders/tv/Terminator%20-%20The%20Sarah%20Connor%20Chronicles/Season%202: verified Bench composer renders, no horizontal overflow, unavailable host remains visible, Send is disabled with the correct blocker after typing