fix(api): return 400 for invalid write bodies

[Lubrsy706]

Fixes #16.

Summary

• add a shared parseJsonBody helper that catches malformed JSON and Zod validation failures
• convert mailbox, send, draft, email update/move, folder, reply, and forward write routes to return 400 for invalid bodies
• add schemas for previously unchecked write bodies such as folder updates and email move requests

Tests

• npm run typecheck