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.

internal/examples/signup/main.go

@@ -26,13 +26,13 @@ func main() {
 		email := c.Field("", via.Required(), via.Email())
 		age := c.Field("", via.Required(), via.Min(13), via.Max(120))
 		// Optional field — only validated when non-empty
-		website := c.Field("", via.Custom(func(val string) error { return nil }), via.Pattern(`^$|^https?://\S+$`, "Must be a valid URL"))
+		website := c.Field("", via.Pattern(`^$|^https?://\S+$`, "Must be a valid URL"))
 
 		var success string
 
 		signup := c.Action(func() {
 			success = ""
-			if !c.ValidateAll(username, email, age, website) {
+			if !c.ValidateAll() {
 				c.Sync()
 				return
 			}
@@ -43,13 +43,13 @@ func main() {
 				return
 			}
 			success = "Account created for " + username.String() + "!"
-			c.ResetFields(username, email, age, website)
+			c.ResetFields()
 			c.Sync()
 		})
 
 		reset := c.Action(func() {
 			success = ""
-			c.ResetFields(username, email, age, website)
+			c.ResetFields()
 			c.Sync()
 		})