[cli] unemail dev — preview server + compat checker

[productdevbook]

Part of #24.

Expand the current `unemail-mailcrab` bin into a proper CLI:

```bash
unemail dev # starts MailCrab + live preview of templates
unemail preview path/to/template.tsx
unemail check path/to/template.tsx # Gmail/Outlook/Apple Mail compatibility lint
unemail send --to=... --template=... # quick one-off
```

Acceptance

• `citty`-based CLI (zero-dep preferred, but citty is unjs-consistent)
• Preview server renders React Email / jsx-email / MJML
• Compat checker uses a rules DB (like jsx-email's) for dark-mode, image blocking, etc.