fix(products): add product description field

[khaira777]

Summary

• add a Description textarea to the Add/Edit Product modal
• bind it to the existing form.description state and API payload
• associate the new textarea with its label for accessibility

Closes #240

Validation

• npm run lint passed with 0 errors (existing warnings remain)
• npm run build:frontend passed
• npm run build passed
• npm audit --audit-level=high passed for root and frontend
• manual E2E QA passed: create a product with a description, save it, reopen it, and verify the description persists

Out-of-scope test notes

• Existing prepaid-payment E2E tests currently fail because the Confirm Payment button remains disabled; the changed file is unrelated to that POS flow.
• The full suite has shown intermittent unrelated failures; focused reruns of the affected tests passed.

[khaira777]

Follow-up PR #243 updates the outdated prepaid checkout E2E tests. The tests were written before configurable payment methods and now correctly select Cash before confirming payment. npm run test:e2e passes 4/4 on PR #243.