Function validate_form 

pub fn validate_form(form: &mut FormConfig)

Walk the form’s fields and apply the basic validation rules:

• Required: empty value on a required field → "This field is required".
• Email: non-empty Email value missing @ → "Invalid email address".
• Number: non-empty Number value that doesn’t parse as f64 → "Must be a valid number".

The first failing rule per field wins; the function never panics or unwraps. Empty optional fields skip the type-specific checks.