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

README.md

@@ -1343,23 +1343,25 @@ markup. The CSP build runs no expressions, so every Alpine directive in
 `static/js/app.js`: `x-data="collapsible"` and `@click="toggle"`, never
 `x-data="{ open: false }"` or `@click="open = !open"`.
 
-A browser test in `internal/server` loads the webhook page and the event log
-under the real policy and checks that: the add entrypoint form stays hidden
-until Add is clicked; for every target type, the targets section's Add shows
-only a choice of type with Next and Cancel, Next shows only that type's fields
-(no url field for `database` or `log`), Cancel at either step closes the form,
-and saving adds the target; a refused target comes back with its form open, the
-values entered and the reason, and after Cancel the next Add starts with an
-empty form and no reason; the Copy button beside an entrypoint URL reads
-"Copied" once clicked; an entrypoint's Edit button shows its edit form in place
-of its description and hides until the form closes, Cancel hides the form and
-drops what was typed, as does leaving the page and going back to it, and Save
-changes the description; of the recent events on the webhook page only the
-newest starts expanded, each expands and collapses, and Open leads to the
-event's own page; an event in the event log expands and collapses, and so do a
-delivery's attempts inside it; and at phone width the menu button opens and
-closes the mobile menu. It also fails if the browser reports a console warning
-or error, an uncaught exception, or anything the policy refused. `make check`
+A browser test in `internal/server` loads the webhook page, its edit pages and
+the event log under the real policy and checks that: the add entrypoint form
+stays hidden until Add is clicked; for every target type, the targets section's
+Add shows only a choice of type with Next and Cancel, Next shows only that
+type's fields (no url field for `database` or `log`), Cancel at either step
+closes the form, and saving adds the target; a refused target comes back with
+its form open, the values entered and the reason, and after Cancel the next Add
+starts with an empty form and no reason; a refused save on the target edit page
+and on the webhook edit page comes back with the reason and every value
+entered; the Copy button beside an entrypoint URL reads "Copied" once clicked;
+an entrypoint's Edit button shows its edit form in place of its description and
+hides until the form closes, Cancel hides the form and drops what was typed, as
+does leaving the page and going back to it, and Save changes the description;
+of the recent events on the webhook page only the newest starts expanded, each
+expands and collapses, and Open leads to the event's own page; an event in the
+event log expands and collapses, and so do a delivery's attempts inside it; and
+at phone width the menu button opens and closes the mobile menu. It also fails
+if the browser reports a console warning or error, an uncaught exception, or
+anything the policy refused. `make check`
 and the image build lint it but do not run it, and `make test` leaves it out
 (its file is built only with the `browser` build tag). Run it with
 `make test-browser` after changing `templates/` or `static/js/`: that builds