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/hooks/__tests__/useAuthOnboarding.test.ts

@@ -14,6 +14,7 @@ vi.mock("../../api", () => ({
 
 vi.mock("../../components/model-onboarding-state", () => ({
   isOnboardingCompleted: (...args: unknown[]) => mockIsOnboardingCompleted(...args),
+  ONBOARDING_FLOW_STEPS: ["ai-setup", "github", "project-setup", "first-task"],
 }));
 
 vi.mock("../../components/onboarding-events", () => ({