refactor: replace legacy delete-button with link-action (#38143)

Removes the legacy `delete-button` handler (`initGlobalDeleteButton`)
and migrates all remaining usages to `link-action` and `show-modal` /
`form-fetch-action`.

Two handlers are adjusted for the new request shape: webauthn key delete
reads `id` from the query, and account deletion returns `JSONError` on
validation failure.

A E2E test ist added to cover one of the use cases.

Suggested in
https://github.com/go-gitea/gitea/pull/38046#discussion_r3414936737.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: bircni <bircni@icloud.com>

tests/integration/html_helper.go

@@ -41,6 +41,7 @@ func (doc *HTMLDoc) Find(selector string) *goquery.Selection {
 
 // AssertHTMLElement check if the element by selector exists or does not exist depending on checkExists
 func AssertHTMLElement[T int | bool](t testing.TB, doc *HTMLDoc, selector string, checkExists T) {
+	t.Helper()
 	sel := doc.doc.Find(selector)
 	switch v := any(checkExists).(type) {
 	case bool: