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>

web_src/css/modules/button.css

@@ -326,11 +326,6 @@
   margin: 0 0.25em 0 0;
 }
 
-.delete-button,
-.delete-button:hover {
-  color: var(--color-red);
-}
-
 /* btn is a plain button without any opinionated styling, it only uses flex for vertical alignment like ".ui.button" in base.css */
 
 .btn {