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

A refused save on the target edit page now shows the edit form again,
with the reason above it and every value submitted, instead of a bare
text page; the status codes are unchanged. The webhook edit page keeps
the submitted name, description and retention the same way, while the
page still reports the stored retention.

Target edits are validated by setTargetFromForm, which newTarget now
uses too, so the add and edit forms accept and refuse the same things.
An empty max_retries keeps the target's own count.

The browser test also saves both edit pages with refused values.

Model: opus-5-5

internal/handlers/target_retries.go

@@ -2,7 +2,6 @@ package handlers
 
 import (
 	"errors"
-	"net/http"
 	"strconv"
 	"strings"
 )
@@ -89,32 +88,3 @@ func retriesErrorMessage(err error) string {
 	return errRetriesInvalid.Error() +
 		", or 0 for fire-and-forget"
 }
-
-// targetMaxRetries reads and validates max_retries from a target edit
-// submission, answering the request with a 400 and reporting false
-// when the value is set but invalid.
-//
-// It and the create path (newTarget) both use parseMaxRetries and
-// retriesErrorMessage, so the two cannot come to disagree about what a
-// valid retry count is. The wording matches the timeout control on
-// the same submission.
-func targetMaxRetries(
-	w http.ResponseWriter,
-	r *http.Request,
-	fallback int,
-) (int, bool) {
-	retries, err := parseMaxRetries(
-		r.PostFormValue("max_retries"), fallback,
-	)
-	if err != nil {
-		http.Error(
-			w,
-			"Invalid max retries: "+retriesErrorMessage(err),
-			http.StatusBadRequest,
-		)
-
-		return 0, false
-	}
-
-	return retries, true
-}