feat: auto-track fields on context for zero-arg ValidateAll/ResetFields

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.

rule.go

@@ -1,10 +1,12 @@
 package via
 
 import (
+	"errors"
 	"fmt"
 	"regexp"
 	"strconv"
 	"strings"
+	"unicode/utf8"
 )
 
 // Rule defines a single validation check for a Field.
@@ -20,7 +22,7 @@ func Required(msg ...string) Rule {
 	}
 	return Rule{func(val string) error {
 		if strings.TrimSpace(val) == "" {
-			return fmt.Errorf("%s", m)
+			return errors.New(m)
 		}
 		return nil
 	}}
@@ -33,8 +35,8 @@ func MinLen(n int, msg ...string) Rule {
 		m = msg[0]
 	}
 	return Rule{func(val string) error {
-		if len(val) < n {
-			return fmt.Errorf("%s", m)
+		if utf8.RuneCountInString(val) < n {
+			return errors.New(m)
 		}
 		return nil
 	}}
@@ -47,8 +49,8 @@ func MaxLen(n int, msg ...string) Rule {
 		m = msg[0]
 	}
 	return Rule{func(val string) error {
-		if len(val) > n {
-			return fmt.Errorf("%s", m)
+		if utf8.RuneCountInString(val) > n {
+			return errors.New(m)
 		}
 		return nil
 	}}
@@ -63,10 +65,10 @@ func Min(n int, msg ...string) Rule {
 	return Rule{func(val string) error {
 		v, err := strconv.Atoi(val)
 		if err != nil {
-			return fmt.Errorf("%s", m)
+			return errors.New("Must be a valid number")
 		}
 		if v < n {
-			return fmt.Errorf("%s", m)
+			return errors.New(m)
 		}
 		return nil
 	}}
@@ -81,10 +83,10 @@ func Max(n int, msg ...string) Rule {
 	return Rule{func(val string) error {
 		v, err := strconv.Atoi(val)
 		if err != nil {
-			return fmt.Errorf("%s", m)
+			return errors.New("Must be a valid number")
 		}
 		if v > n {
-			return fmt.Errorf("%s", m)
+			return errors.New(m)
 		}
 		return nil
 	}}
@@ -99,7 +101,7 @@ func Pattern(re string, msg ...string) Rule {
 	compiled := regexp.MustCompile(re)
 	return Rule{func(val string) error {
 		if !compiled.MatchString(val) {
-			return fmt.Errorf("%s", m)
+			return errors.New(m)
 		}
 		return nil
 	}}
@@ -115,7 +117,7 @@ func Email(msg ...string) Rule {
 	}
 	return Rule{func(val string) error {
 		if !emailRegexp.MatchString(val) {
-			return fmt.Errorf("%s", m)
+			return errors.New(m)
 		}
 		return nil
 	}}