feat: onboarding source awareness in admin UI (#145)
* feat(types): add source and hostingMode to onboarding types
Add OnboardingFieldSource, HostingMode, and AdminOnboardingFieldsResponse
types. Update getOnboardingFields client to return new response shape.
Update hook to extract fields and hostingMode. Update all mocks to include
source field.
* feat(admin): show source badges and SaaS read-only state for platform fields
Platform fields display a "Platform" badge in the admin onboarding list.
In SaaS hosting mode, platform field controls (edit, delete, reorder) are
disabled with an explanatory note. In selfhosted mode all fields remain
fully editable. Adds platform age_confirmation to mock data.
* style: fix Prettier formatting
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