feat(FN-2403): add project setup step to onboarding flow

- Introduce a new "project-setup" onboarding step between GitHub and first-task actions
- Gate first-task navigation on project selection and add contextual setup-wizard guidance
- Centralize ordered onboarding step definitions in shared state and reuse them for navigation/progress labels
- Extract ModelOnboardingModal styles into a dedicated component CSS file and update tests/mocks for the new step order

packages/dashboard/app/components/__tests__/App.test.tsx

@@ -132,6 +132,7 @@ vi.mock("../../components/model-onboarding-state", () => ({
   getOnboardingCompletedAt: (...args: unknown[]) => mockGetOnboardingCompletedAt(...args),
   getSkippedSteps: (...args: unknown[]) => mockGetSkippedSteps(...args),
   getStepData: (...args: unknown[]) => mockGetStepData(...args),
+  ONBOARDING_FLOW_STEPS: ["ai-setup", "github", "project-setup", "first-task"],
 }));
 
 // Mock CustomModelDropdown for onboarding modal tests