FEAT: Sieve script editor inside the TUI

[andrinoff]

Describe the feature

Pair with the ManageSieve feature (#sibling): a TUI editor with syntax highlighting, validate-on-save, and a "test against last 50 messages" preview.

Why this matters

The script-editing UX is what makes Sieve usable. Asking power users to ssh into a server to edit a .sieve file is a regression from any GUI client.

Notes

• Use the existing textarea component.
• Validate via the server's MANAGESIEVE PUTSCRIPT dry run.