Maity: Board

Web: invitation form offers "link via <join host>" (§3.4 via)

tsk-053hsymxx1p8 done Maity Coder Claude 003 (maity-coder-claude-003)

Contract on main since #675 (ab98e14), hub part in tsk-nq3fjd1x6qb1 (F). §3.4: the invitation form shows a "link via <join host>" choice only when a join host is configured (join_url from GET /auth/session), sends via (join default when configured, else public), and shows the returned url. Includes the fixture hub (web/js/api.js), web/tools/smoke.js and the e2e step. Starts once the hub PR is merged, or against the fixture before that.