feat(e2e): add Playwright end-to-end tests for authentication, GRN, production, stock transfers, and adjustments

- Introduced Playwright configuration for e2e testing.
- Implemented authentication tests to validate login functionality.
- Created tests for GRN (Goods Receipt Note) to ensure proper stock handling.
- Developed production run tests to verify lifecycle and stock posting.
- Added stock transfer tests to check movement between warehouses.
- Implemented stock adjustment tests for positive and negative adjustments.
- Established API seeder for test data setup and verification.
- Enhanced utility functions for UI interactions and response handling.

Testing/e2e/.env.e2e.example

@@ -0,0 +1,14 @@
+# Copy to .env.e2e and fill in real values. Never commit .env.e2e.
+
+# Frontend origin the browser navigates to (Next.js proxies /api/v1/* server-side from here).
+E2E_BASE_URL=http://localhost:3000
+
+# Backend origin, used only to build absolute API paths in error messages / docs; all
+# actual requests go through E2E_BASE_URL's same-origin /api/v1 proxy.
+E2E_API_URL=http://localhost:5224
+
+# Credentials for a real AuthHex-backed user with access to Receiving, Production, and
+# Stock modules. AuthHex is an external identity provider (see docs/11-BACKEND-PHASE1.md
+# §2.0) - there is no local seed for this account, it must already exist upstream.
+E2E_ADMIN_EMAIL=e2e-tester@example.com
+E2E_ADMIN_PASSWORD=change-me