Orgs: let any user create an organization and become its admin

Organization writes were superadmin-only, so standing up a tenant needed
an out-of-band superadmin. Creating one is now self-service, and an admin
manages the org they belong to.

- POST /api/orgs is open to any authenticated user. A creator who isn't a
  superadmin must have no organization yet (a single-valued membership
  relation means a second one would abandon the first), and is promoted to
  the new org's admin and first member in the same request. If that
  promotion fails the org is rolled back, so it is never left stranded
  with nobody able to administer it. Superadmins still create tenants
  without joining them.
- PATCH/DELETE are manager-gated and scope an admin to their own org. An
  admin deletes theirs only as its sole member: they are detached and
  demoted to a plain user before the record goes, so the org is empty when
  it is removed. Other members still block deletion with a 409.
- /api/me now carries organization + organizationName, which the clients
  need to tell "no org yet" from "org you administer".

The panel, Web App (new OrgManager.vue in Settings) and Phone App (new
_OrganizationSection) all mirror the server's gates rather than
re-deciding them. The Phone App cached its role at login and gates the
Users tab on it, so AuthService.adoptRole refreshes that from the profile
instead of making a freshly promoted admin sign in again.

Covered by orgs_test.go, which drives the real handler + middleware chain
against a stand-in PocketBase: promotion, the already-a-member refusal,
superadmin staying unattached, the rollback, own-org scoping, the
detach-and-demote, and the blocking-member 409.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -57,6 +57,7 @@ export/import).
   export/import, and an account-deletion state machine.
 - **Organizations & roles** — multi-tenant `user` / `admin` / `superadmin`
   roles; admins manage users within their own organization, superadmins span all.
+  Any user without an organization can create one and becomes its admin.
 - **Per-user ownership & sharing** — each car has an owner and can be shared with
   other users as read or write; the UI mirrors the server's access checks.
 - **Integrations** — per-user connectors under a superadmin → org-admin → user
@@ -86,7 +87,9 @@ against PocketBase on each call, so a role change or a deletion takes effect
 immediately. Tokens are stateless, so there is no per-device session list;
 changing an account's password rotates its token key and invalidates every token
 already issued. Access to cars/records is gated by per-user ownership and shares;
-user management requires the admin or superadmin role.
+user management requires the admin or superadmin role. Creating an organization
+is the one management action open to a plain user — it promotes them to admin of
+the organization they just created.
 
 ## Domain (from `Car Service.xlsx`)