Keep what was typed when a target or webhook edit is refused (closes #381)

A refused save on the target edit page answered with a bare text page, losing the form and everything typed, and the webhook edit page came back with the stored values instead of the submitted ones. A refused target edit now shows the edit form again with the reason above it and every value submitted, with the same status codes as before; a refused webhook edit keeps the submitted name, description and retention. Target edits use the same validation as new targets, with no second copy; an encoding or database failure stays a logged 500. The browser test covers a refused save on both pages, and its main function is now a plain list of checks.

Model: opus-5-5

internal/handlers/ui_copy_test.go

@@ -396,21 +396,21 @@ func TestTargetFormMaxRetriesCopyMatchesBehaviour(t *testing.T) {
 	)
 
 	// A slack target exercises the same max_retries field while needing
-	// only Config.URL from the edit template, so the test data stays
-	// minimal. The Target key mirrors the field names the template reads
-	// off the handler's view value.
+	// only a URL from the edit template, so the test data stays
+	// minimal. The Target and TargetForm keys mirror the field names
+	// the template reads off the handler's values.
 	editBody := renderPage(
 		t, h, sess, "target_edit.html", map[string]any{
 			dataKeyWebhook: webhook,
 			"Target": map[string]any{
-				"ID":         "tg-1",
-				"Name":       "t",
-				"Type":       "slack",
-				"Active":     true,
-				"MaxRetries": 3,
-				"Config": map[string]any{
-					"URL": "https://hooks.slack.com/services/x",
-				},
+				"ID":     "tg-1",
+				"Name":   "t",
+				"Type":   "slack",
+				"Active": true,
+			},
+			"TargetForm": map[string]any{
+				"URL":        "https://hooks.slack.com/services/x",
+				"MaxRetries": "3",
 			},
 			dataKeyError: "",
 		},