Adds native HTML5 drag-and-drop so users can move recipient email
address chips between the To, CC, and BCC fields in the composer.
Chips dragged onto the Cc/Bcc toggle buttons auto-reveal the hidden
field and place the chip there.
Clicking an email address in the viewer already shows a contact detail
sidebar. An "Edit" button now appears there (for known contacts) that
navigates directly to the contact edit form via the existing URL-param
intent system (?contactId=…&view=edit), removing the need to open the
Contacts page manually and search for the contact.
