Seed from go-template-repo, renamed to simplexcalc

The template's files at a77fd30, without its history or LICENSE, after
script/rename simplexcalc.

Model: opus-5-5

internal/handlers/errors.go

@@ -0,0 +1,13 @@
+package handlers
+
+import "errors"
+
+var (
+	// errBadWidgetName is a rejected form value, not a fault. It exists
+	// so that h.fail always has a non-nil error to log: a rejection
+	// with no error recorded is a rejection nobody can explain later.
+	errBadWidgetName = errors.New("widget name is empty or too long")
+
+	// errBadWidgetSize is a size field that is not a byte count.
+	errBadWidgetSize = errors.New("widget size is not a byte count")
+)