feat: add email threading and branded templates

[mpge]

Summary

• Add Email_Threading class that hooks into wp_mail to inject Message-ID, In-Reply-To, and References headers for proper email client threading
• Add Branded_Email_Template class for rendering HTML emails with configurable logo, accent color, footer text, and company name
• Branding settings stored in escalated_settings table via the Setting model
• Registered in the plugin bootstrap via class-escalated.php

Test plan

• Verify Message-ID generation for tickets and replies
• Verify In-Reply-To and References headers are added for reply emails
• Verify headers are not added when no ticket context exists
• Verify context resets after email is sent
• Verify branded template renders with logo, accent color, footer, and company name
• Verify invalid accent color falls back to default
• Verify branding settings can be stored and retrieved