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>

routers/web/user/setting/security/webauthn.go

@@ -132,8 +132,7 @@ func WebauthnDelete(ctx *context.Context) {
 		return
 	}
 
-	form := web.GetForm(ctx).(*forms.WebauthnDeleteForm)
-	if _, err := auth.DeleteCredential(ctx, form.ID, ctx.Doer.ID); err != nil {
+	if _, err := auth.DeleteCredential(ctx, ctx.FormInt64("id"), ctx.Doer.ID); err != nil {
 		ctx.ServerError("GetWebAuthnCredentialByID", err)
 		return
 	}