feat(qsl): set the QSL route per direction from the QSL Manager

The paper form had one "Via", which since the QSL_VIA split writes
QSL_SENT_VIA. QSL_RCVD_VIA had no way in at all outside the per-QSO editor —
a field with a column, an import and an export, and nothing to fill it.

Each direction now has its own route dropdown, sitting with the status and
date it belongs to: how the card was sent, beside Sent; how it came back,
beside Received. Both apply to the whole selection, which is the shape of the
job — a stack confirmed in one go usually arrived the same way, a bureau
delivery being exactly that, and doing it one QSO at a time in the editor was
the only option before.

Both store the ADIF enumeration (B/D/E), and neither touches QSL_VIA — that
holds the manager's callsign and belongs to the QSO, not to a stack of cards
being confirmed together.

The two columns were already available in the paper list: it renders through
RecentQSOsGrid, which gained them with the field itself.

frontend/wailsjs/go/models.ts

@@ -2739,6 +2739,7 @@ export namespace main {
 	    sent_date: string;
 	    rcvd_date: string;
 	    via: string;
+	    rcvd_via: string;
 	    notes: string;
 	    comment: string;
 	
@@ -2753,6 +2754,7 @@ export namespace main {
 	        this.sent_date = source["sent_date"];
 	        this.rcvd_date = source["rcvd_date"];
 	        this.via = source["via"];
+	        this.rcvd_via = source["rcvd_via"];
 	        this.notes = source["notes"];
 	        this.comment = source["comment"];
 	    }