No way to delete a team — add team deletion (guarded against deleting issues)

The Teams screen can create teams (and toggle public) but there's no way to delete one. issue.team_id is ON DELETE CASCADE, so a naive delete would silently destroy every issue in the team — needs a guard.

Backend (tesks):

• WorkspaceAdminService.deleteTeam(id) — require SETTINGS_MANAGE; refuse if the team has any issues (count issue by team_id/tenant), with a clear message ("move or delete its N issues first"); else delete from team where id/tenant.
• DELETE /api/w/{slug}/teams/{id} in WorkspaceApiController.

Frontend (tesks-ui):

• lib/api.ts deleteTeam(slug, id); deleteTeamAction in lib/actions/workspace.ts.
• Teams page: a Delete control on empty teams (issueCount === 0) for users who can manage settings, with confirmation. Non-empty teams show no delete (move issues out first; the count is live so it appears once empty).