Add moduleNameMapper for shared lib mocks in Jest

Create dedicated mock files for shared/lib modules and map them
in moduleNameMapper for reliable module resolution in CI.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

mobile/__mocks__/shared-lib-referral.js

@@ -0,0 +1,11 @@
+// Mock for shared/lib/referral.ts
+module.exports = {
+  initiateReferral: jest.fn(() => Promise.resolve()),
+  getPendingReferral: jest.fn(() => Promise.resolve(null)),
+  getReferralCount: jest.fn(() => Promise.resolve(0)),
+  getReferralInfo: jest.fn(() => Promise.resolve(null)),
+  calculateReferralScore: jest.fn(() => 0),
+  getReferralStats: jest.fn(() => Promise.resolve({ totalReferred: 0, pendingCount: 0, confirmedCount: 0 })),
+  getMyReferrals: jest.fn(() => Promise.resolve([])),
+  subscribeToReferralEvents: jest.fn(() => jest.fn()),
+};