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>

templates/user/settings/security/webauthn.tmpl

@@ -16,7 +16,7 @@
 					</div>
 				</div>
 				<div class="item-trailing">
-					<button class="ui red tiny button delete-button" data-modal-id="delete-registration" data-url="{{$.Link}}/webauthn/delete" data-id="{{.ID}}">
+					<button class="ui red tiny button link-action" data-modal-confirm="#delete-registration" data-url="{{$.Link}}/webauthn/delete?id={{.ID}}">
 					{{ctx.Locale.Tr "settings.delete_key"}}
 					</button>
 				</div>
@@ -30,7 +30,7 @@
 		</div>
 		<button id="register-webauthn" class="ui primary button">{{svg "octicon-key"}} {{ctx.Locale.Tr "settings.webauthn_register_key"}}</button>
 	</div>
-	<div class="ui g-modal-confirm delete modal" id="delete-registration">
+	<div class="ui small modal" id="delete-registration">
 		<div class="header">
 			{{svg "octicon-trash"}}
 			{{ctx.Locale.Tr "settings.webauthn_delete_key"}}