BUG: Composer does not validate email addresses in To/Cc/Bcc

[andrinoff]

Describe the bug: Composer accepts invalid email addresses. Can send to "notanemail" or "user@" without validation. SMTP rejects later with unclear error.

To reproduce:

1. Compose new email
2. Enter "invalid" in To field
3. Send - SMTP error appears
4. Should validate before send attempt

Expected behavior: Validate email format in real-time or before send:

• Check for @ symbol
• Validate domain part
• Show error: "Invalid email address: invalid"

Additional context:

• File: tui/composer.go
• Use mail.ParseAddress() for validation
• Show inline error in red
• Severity: Low - UX issue
• Complexity: Easy - email validation