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/delete_user_test.go

@@ -13,7 +13,10 @@ import (
 	repo_model "gitea.dev/models/repo"
 	"gitea.dev/models/unittest"
 	user_model "gitea.dev/models/user"
+	"gitea.dev/modules/test"
 	"gitea.dev/tests"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func assertUserDeleted(t *testing.T, userID int64) {
@@ -34,7 +37,8 @@ func TestUserDeleteAccount(t *testing.T) {
 	session := loginUser(t, "user8")
 	urlStr := "/user/settings/account/delete?password=" + userPassword
 	req := NewRequest(t, "POST", urlStr)
-	session.MakeRequest(t, req, http.StatusSeeOther)
+	resp := session.MakeRequest(t, req, http.StatusOK)
+	assert.NotEmpty(t, test.ParseJSONRedirect(resp.Body.Bytes()).Redirect)
 
 	assertUserDeleted(t, 8)
 	unittest.CheckConsistencyFor(t, &user_model.User{})
@@ -46,8 +50,8 @@ func TestUserDeleteAccountStillOwnRepos(t *testing.T) {
 	session := loginUser(t, "user2")
 	urlStr := "/user/settings/account/delete?password=" + userPassword
 	req := NewRequest(t, "POST", urlStr)
-	session.MakeRequest(t, req, http.StatusSeeOther)
-
+	resp := session.MakeRequest(t, req, http.StatusBadRequest)
+	assert.NotEmpty(t, test.ParseJSONError(resp.Body.Bytes()).ErrorMessage)
 	// user should not have been deleted, because the user still owns repos
 	unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
 }