Email API: attach populated ChangeDetection.html report (base64)

Email API: send required FileName and AttachmentBase64 placeholders

Use manager email API for notifications (multipart/form-data); SMTP fallback

Revert Resend; use SMTP only, custom email API to be integrated later

Use Resend HTTPS API for email (HF Spaces); SMTP fallback for local

Rework email notifications with explicit send actions.

Switch email from SMTP_SSL:465 to STARTTLS:587, restore sender, show failure feedback

Full codebase audit: fix critical perf bug, security, error handling, dead code

Add email notification feature with Notify checkbox, SMTP backend, and HTML template