Implement VippsOrgCostConfig repository and subscription gate logic

Define a VippsOrgCostConfig Dart model with fromJson/toJson using explicit field mapping (avoid code generation for this small model to keep dependencies minimal). The repository method isVippsEnabledForOrg should perform a single SELECT with .eq('org_id', orgId).maybeSingle() — maybeSingle() returns null rather than throwing when no row exists, which maps cleanly to the false gate case. For the Riverpod provider, use AsyncNotifier or FutureProvider.family(orgId) depending on whether mutation (activateSubscription) is needed from the same provider — prefer AsyncNotifier if mutation state is required. In the authentication flow, call isVippsEnabledForOrg in the organization selection step so the Vipps button is never shown to users whose org has not activated the subscription — this prevents user confusion and avoids initiating a Vipps OAuth flow that would ultimately fail.

Write unit tests using flutter_test and mocktail. Mock the Supabase client at the repository constructor boundary. Test cases: (1) isVippsEnabledForOrg with active=true row returns true, (2) active=false returns false, (3) no row found returns false, (4) Supabase throws PostgrestException — repository wraps it in VippsConfigException, (5) getCostConfig deserializes all fields correctly from a mock JSON response, (6) activateSubscription sends a correct PATCH request and returns updated model. Use ProviderContainer in widget tests to verify that vippsEnabledForOrgProvider emits correct AsyncValue states.

Minimum 90% line coverage on repository class.