feat(auth): signup org step (create or join, conditional on config) #3685

Description

Add an organization step to the signup flow, conditional on the organizations config from GET /api/auth/config.

Details

When organizations.enabled === true:

After email/password/name step, show step 2:

• Backend signup response indicates if domain matched an existing organization
  • Match found: "Join Acme Inc?" — simple confirm button
  • No match: form to create organization (name, slug)

When organizations.enabled === false:

• No extra step — signup works as today
• Organization created silently in backend

Implementation

• The auth/config is already fetched at boot — use authStore.serverConfig.organizations.enabled
• Signup view conditionally renders the organization step
• Uses the signup response to determine match/no-match

Acceptance criteria

• Organization step shown only when organizations enabled
• Domain match: "Join organization" confirmation
• No match: organization creation form
• Organizations disabled: no visible change to signup