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/application/usecase/webmail_compose_test.go

@@ -402,6 +402,28 @@ func TestDeleteFromTrashExpunges(t *testing.T) {
 	}
 }
 
+// "Discard" in the composer promises the draft is gone. Delete would leave it
+// in Trash, so discarding removes it outright - and only ever from Drafts.
+func TestDiscardingADraftRemovesItForGood(t *testing.T) {
+	mailboxID := uuid.New()
+	repo := &stubWebmailRepo{mailboxID: mailboxID.String()}
+	uc := &WebmailUseCase{
+		repo:      repo,
+		auth:      &stubAuthRepo{account: &port.MailboxAuth{ID: mailboxID, EmailAddress: "me@example.test"}},
+		localHost: "mail.example.test",
+	}
+
+	if err := uc.DiscardDraft(context.Background(), "me@example.test", 9); err != nil {
+		t.Fatalf("discard: %v", err)
+	}
+	if len(repo.expunged) != 1 || repo.expunged[0] != 9 {
+		t.Errorf("the draft was not expunged: %v", repo.expunged)
+	}
+	if len(repo.moved) != 0 {
+		t.Errorf("a discarded draft was moved instead of removed: %v", repo.moved)
+	}
+}
+
 // --- moving between folders ---------------------------------------------
 
 // Filing a message somewhere else is the one ordinary mail action the webmail