Validate hex colors when creating/editing labels (#34623)

Resolves #34618.

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/shared/label/label.go

@@ -0,0 +1,26 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package label
+
+import (
+	"code.gitea.io/gitea/modules/label"
+	"code.gitea.io/gitea/modules/web"
+	"code.gitea.io/gitea/services/context"
+	"code.gitea.io/gitea/services/forms"
+)
+
+func GetLabelEditForm(ctx *context.Context) *forms.CreateLabelForm {
+	form := web.GetForm(ctx).(*forms.CreateLabelForm)
+	if ctx.HasError() {
+		ctx.JSONError(ctx.Data["ErrorMsg"].(string))
+		return nil
+	}
+	var err error
+	form.Color, err = label.NormalizeColor(form.Color)
+	if err != nil {
+		ctx.JSONError(ctx.Tr("repo.issues.label_color_invalid"))
+		return nil
+	}
+	return form
+}