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_mail.go

@@ -283,6 +283,20 @@ func (uc *WebmailUseCase) Delete(ctx context.Context, address, folder string, ui
 	}
 }
 
+// DiscardDraft removes a draft for good.
+//
+// Delete would move it to Trash, which is right for a message someone might
+// want back and wrong for "Discard": the composer's button promises the draft
+// is gone, and one waiting in Trash is a draft that is not. Only Drafts is
+// reachable from here, so this cannot be used to expunge anything else.
+func (uc *WebmailUseCase) DiscardDraft(ctx context.Context, address string, uid uint32) error {
+	mailboxID, err := uc.mailboxID(ctx, address)
+	if err != nil {
+		return err
+	}
+	return uc.repo.Expunge(ctx, mailboxID, "Drafts", uid)
+}
+
 // SaveDraft files the message being composed into the Drafts folder.
 //
 // Nothing did this before: the composer displayed "draft saved automatically"