Implement error handling and validation for input fields

[BowenAnna]

User Story Description
As a manager, I want the system to validate inputs such as week selections and dish assignments, so that I can avoid errors like overlapping weeks, duplicate dishes, or invalid selections, and receive clear error messages for any invalid input.

Steps to Follow (optional)

• Step Getting Started: 📜 Review the Chingu Agile Methodology #1
• Step Getting Started: 🤔 Start thinking about project ideas & the technical stack #2
• Additional steps as necessary

Additional Considerations
Any supplemental information including unresolved questions, links to external resources, screenshots, etc.