Fields created via Context.Field are now tracked on the page context,
so ValidateAll() and ResetFields() with no arguments operate on all
fields by default. Explicit field args still work for selective use.
Also switches MinLen/MaxLen to utf8.RuneCountInString for correct
unicode handling and replaces fmt.Errorf with errors.New where
format strings are unnecessary.
Introduces Field, Rule, ValidateAll, ResetFields, and AddError for
declarative input validation. Includes built-in rules (Required,
MinLen, MaxLen, Min, Max, Email, Pattern, Custom) and a signup
example exercising the full API surface.