[elliotBraem]

Problem

Starting a community still ended on the Web Address field and
"An organization already uses this address. Choose another one." even
though #466 and #468 merged. Those PRs made the form tolerate a slug
collision with the applicant's own organization, but kept the
slug-input-first design:

• Name prefill only happened when arriving with ?org= — every other
  entry point (nav, landing page, dashboard next steps, proposals page)
  links to bare /apply, so nothing was prefilled.
• The session's active organization was never consulted; linking
  only kicked in after the user typed/derived a matching slug, and the
  checkSlug query raced the ownership lookup in between, flashing the
  "address is taken" error.

What changed

• /apply resolves the applying organization up front: the ?org=
  parameter first, otherwise the session's active organization.
• Eligibility = active status, not personal, no community already bound
  (communityProposalId), and the applicant is an owner (via
  listMembers).
• Eligible org → name prefilled (editable), slug locked to the org
  slug, payload submitted with orgId, and the web address rendered as
  static text below the name (<slug>.<gateway>), not an input.
  checkSlug never runs, so the taken-address error is impossible.
• Ineligible selection (personal workspace, already a community, or
  not the owner) → a "Choose an organization" prompt linking to the
  organizations page, instead of a confusing free-form slug field.
• No organization selected → the plain free-form form with the
  existing availability checks (the nav "Start a community" bootstrap
  path for applicants with no organization).
• A loading gate removes the query-race flashes (org/members loading ±
  checkSlug returning "taken" in the gap).
• The "Use a different address" unlock and its i18n key were removed;
  added apply.organization.pickPrompt in en/es/fr/zh.

Tests

• Split the suite into org-linked mode (prefilled name, locked address
  rendered as text, no checkSlug, orgId in the payload, default
  active-org path and ?org= path) and free-form mode (taken-slug
  error, no orgId, revision/legacy-org retention).
• New prompt tests for the ineligible-org and not-owner cases.
• pnpm run typecheck, pnpm run lint, and the full ui suite
  (1129 tests) pass.