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>

services/forms/user_form.go

@@ -419,17 +419,6 @@ func (f *WebauthnRegistrationForm) Validate(req *http.Request, errs binding.Erro
 	return middleware.Validate(errs, ctx.Data, f, ctx.Locale)
 }
 
-// WebauthnDeleteForm for deleting WebAuthn keys
-type WebauthnDeleteForm struct {
-	ID int64 `binding:"Required"`
-}
-
-// Validate validates the fields
-func (f *WebauthnDeleteForm) Validate(req *http.Request, errs binding.Errors) binding.Errors {
-	ctx := context.GetValidateContext(req)
-	return middleware.Validate(errs, ctx.Data, f, ctx.Locale)
-}
-
 // PackageSettingForm form for package settings
 type PackageSettingForm struct {
 	Action   string