fix(webmail): Discard really discards the draft

The button was a link to the inbox, so the autosaved draft stayed in Drafts.
It now removes the stored draft for good (not into Trash), stops the autosave,
and cleans up a save that was already in flight.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

services/protocols/internal/presentation/http/mail_api.go

@@ -300,6 +300,8 @@ func (m *mailAPI) handleDelete(w http.ResponseWriter, r *http.Request) {
 	var body struct {
 		Folder string `json:"folder"`
 		UID    uint32 `json:"uid"`
+		// DiscardDraft removes a draft for good instead of moving it to Trash.
+		DiscardDraft bool `json:"discard_draft"`
 	}
 	if err := json.NewDecoder(http.MaxBytesReader(w, r.Body, 4096)).Decode(&body); err != nil {
 		writeError(w, http.StatusBadRequest, "BAD_REQUEST", "Invalid JSON")
@@ -313,6 +315,15 @@ func (m *mailAPI) handleDelete(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if body.DiscardDraft {
+		if err := m.useCase.DiscardDraft(r.Context(), session.Email, body.UID); err != nil {
+			m.fail(w, err)
+			return
+		}
+		writeJSON(w, http.StatusOK, map[string]bool{"deleted": true})
+		return
+	}
+
 	if err := m.useCase.Delete(r.Context(), session.Email, body.Folder, body.UID); err != nil {
 		m.fail(w, err)
 		return