Enable quantity updates for basket items on the Baskets page

[KyleMcMaster]

Summary

Implement functionality to allow users to increase or decrease the quantity of items in their basket directly from the Baskets page.

Details

• Users should be able to increment or decrement the quantity for each basket item.
• Changes should be validated on both the front-end and back-end (e.g., ensure quantities are positive integers, do not exceed available stock, etc.).
• Provide clear user feedback for invalid inputs (e.g., entering a quantity less than 1 or exceeding stock).
• Follow existing coding standards and best practices for F#, Entity Framework, and Falco framework as used in FShopOnWeb.
• Implementation should include comprehensive unit tests to cover validation, business logic, and user feedback scenarios.

Acceptance Criteria

• Users can increase or decrease quantities for each basket item on the Baskets page.
• Invalid inputs trigger appropriate feedback messages.
• All updates are properly reflected in the basket and persisted in the database.
• Code adheres to repository standards for F#, Entity Framework, and Falco.
• All new logic is covered by unit tests.