Recipient suggestions from contacts in the mail app #3850

Initial Checklist

• I understand this is a user story and questions should be posted in the Community Discussions
• I searched issues and couldn’t find anything (or linked relevant results below)

Part of #3844

User Story

As a mail user, I want recipient suggestions from my contacts while composing an email so that I can address people quickly and without typos.

User Value

Picking recipients from the address book is the most frequent everyday use of contacts. The groupware API already supports filtering contacts by name and email.

Acceptance Criteria

• Typing in To / Cc / Bcc shows matching contacts (name and email) from all address books of the account
• Suggestions can be picked with mouse and keyboard
• Contacts with multiple email addresses offer each address
• Suggestions don't block typing; free input of any valid address is still possible
• Requests are debounced and cancelled when the input changes
• Unit tests cover the suggestion lookup