Name people by display name instead of their email address #324

Done in 9f51053 (2026-09-19). People had no name, only their sign-in address, so tasks, history, proposals and assignee lists named them by email, to other members and to agents.

• UserAccount.DisplayName (migration AddUserDisplayName, max 80); existing accounts are backfilled with the part of their address before the @ (checked on LocalDB), new accounts start the same way.
• Editable as "Name" in Settings.
• Assignees, task creators and assignees, task history actors, page history authors and proposal authors and reviewers use the display name. The proposal author filter matches name or address.
• The address stays where it is needed: sign-in, inviting members, the owner's member list, the account's own data export, and the account menu.
• The interim PublicName shortening in the v2 tools (#316) is removed.

Deploying needs the migration applied before the new build serves requests.