Organizations: join and leave

[Adron]

Part of #80 — Organizations parity gaps

/help/organizations covers Joining organizations (browse public organizations and join) and
Leaving an organization. Android can browse and create but not join or leave.

• Determine the real contract for joining — POST /api/user/organizations is the likely candidate
  but is undocumented in the spec; capture the live request before implementing.
• Leave, with a confirmation, and correct behaviour for the last remaining owner.

Tests: join/leave round-trips; last-owner guard.

---

Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.

[Adron]

Implemented in #109 (branch issue/81-org-join-leave). Closing here — review and any follow-up happens on the PR.

The open question in this issue is answered: POST /api/user/organizations is join, not create-under-user. Body is {"organizationId":"<id>"}, returns 201 {"message":"Joined organization successfully","membership":{…}}, with 409 conflict when already a member.

There is no dedicated leave endpoint. Leaving is expressed as removing your own membership via DELETE /api/organizations/{id}/members/{yourUserId}. That works, but it means leave depends on SessionStore.userId being present — worth confirming whether a DELETE /api/user/organizations/{id} exists or is planned, since a members/me route would be sturdier.

Last-owner behaviour confirmed live: the server returns 400 {"error":"Cannot remove the last owner","code":"bad_request"}. The UI detects the case client-side and explains it without offering a destructive action, falling back to mapping that exact server error to the same explanation.

Also fixed along the way: GET /api/organizations/{id} reports membership as userRole, never role — the existing DTO read only role, so the detail screen's role was always null. And GET /api/organizations/{id}/members is members-only (403), so that request is now skipped for non-members rather than surfacing a permission error.

Note for #83: the detail overflow still offers Edit/Delete to non-members. Membership is now known, so gating them is a small change — but it is roles/visibility work, so it was left there.